Slang

Like people everywhere, wizards pepper their speech with slang expressions and interjections. Some of these are the same as those used by their Muggle counterparts, but others are more specific to the Wizarding World. Some are even specific to the particular characters and their shared experiences (e.g. “do a Crouch”.)

There are a few words included which are considered swear words, including “damn” and “hell,” but the worst of such language is not actually transcribed in the text. It is alluded to, presumably to keep the books appropriate for younger readers. Ron seems to be particularly in the habit of using bad language.

Ron caught up with them five minutes later, in a towering rage.
“D’you know what that –” (he called Snape something that made Hermione say “Ron!”) “– is making me do? I’ve got to scrub out the bedpans in the hospital wing. Without magic!” He was breathing deeply, his fists clenched (PA9).

and

Ron told Malfoy to do something that Harry knew he would never have dared say in front of Mrs. Weasley (GF9).

Ron’s most famous slang-ish moment comes in Divination class:

“Oh Professor, look! I think I’ve got an unaspected planet! Oooh, which one’s that, Professor?”
“It is Uranus, my dear,” said Professor Trelawney, peering down at the chart.
“Can I have a look at Uranus too, Lavender?” said Ron.
Most unfortunately, Professor Trelawney heard him, and it was this, perhaps, that made her give them so much homework at the end of the class (GF13).

Here are slang expressions used by witches and wizards:

“The odds must be very, very long. Even Bagman wouldn’t have bet — ” (OP8)
Cornelius Fudge used this at Harry’s disciplinary hearing to indicate how unlikely he thought it that dementors would just happen to run across the only registered wizarding person in the whole of Little Whinging. The reference is to compulsive gambler Ludo Bagman.

“blood traitor” (OP6)
A pureblood witch or wizard who doesn’t show prejudice to those who aren’t pureblood.

“cat’s among the pixies” – Mrs. Figg (OP2)
Synonymous with the Muggle cliché “cat among the pigeons“.

“come over so Dursley-ish” – Harry, privately (OP11)
Referring to the Dursleys‘ intensely negative attitude toward the magical world in general. Harry used this in his own mind when wondering why Seamus Finnigan’s witch mother would consider withdrawing Seamus from Hogwarts, just before he and Seamus had their argument about the Daily Prophet‘s smear campaign.

“common goblin” – Winky (GF8)
used by Winky to describe Dobby’s unbecoming behaviour:

“…You goes racketing around like this, Dobby, I says, and next thing I hear you’s up in front of the Department for the Regulation and Control of Magical Creatures, like some common goblin.”

“creature of dirt” Portrait of Walburga Black (OP6)
This and other references to filth and mud are essentially synonymous with the epithet “Mudblood“.

“crying over spilt potion” – Mrs. Figg (OP2)
Synonymous with the Muggle cliché “crying over spilt milk“, meaning that the damage has been done, can’t be undone, and fussing about it won’t help.

“do a Crouch” – Ron (GF29)
Ron is suggesting with this phrase that Percy Weasley might betray his own family in the name of following rules, in the same way that Barty Crouch Sr disowned his own son with barely a fair trial.

“do a Weasley” (OP30)
Expression among Hogwarts students, referring to the twins’ spectacular departure.

“eat dung” – Ron to Malfoy (GF11)
Essentially no less rude than the more common expression which wouldn’t be used in a children’s book.

“eat slugs” – also Ron to Malfoy, shortly before the slug curse backfired (CS6)
Possibly using this insult gave Ron the idea for the slug-vomiting charm he tried to use a little while later. “Eat slugs” isn’t the actual incantation, of course; it’s just a slang expression.

“fell off the back of a broom” (OP2)
Analogous to the Muggle expression “fell off the back of a truck“, indicating that an item is of questionable origin and probably stolen (probably from some bulk shipment to a retail outlet or similar). Mrs. Figg uses this while raging about Mundungus Fletcher’s dereliction of duty – he’d left Harry without cover because he’d just had an opportunity to acquire a load of cauldrons that had ‘fallen off the back of a broom’ – the dodgy cauldrons he later tried to stash at Grimmauld Place (OP6).

“Gallopin’ Gorgons” – Hagrid (PS4)
A Gorgon is a creature from Greek mythology and possibly, in the Wizarding World, a real creature – if the poem in the Book of Spells is true (BoS). Here’s what the Muggle Encyclopedia Mythica has to say about them:

In Greek mythology a Gorgon is a monstrous feminine creature whose appearance would turn anyone who laid eyes upon it to stone…The Gorgons are monstrous creatures covered with impenetrable scales, with hair of living snakes, hands made of brass, sharp fangs and a beard. They live in the ultimate west, near the ocean, and guard the entrance to the underworld (Encyclopedia Mythica).

“gulping gargoyles” (GF9)
Expression of surprise. There really are gargoyles in the magical world, if one counts the stone statues outside the staffroom at Hogwarts (OP17).

“hold your hippogriffs!” – Hagrid (OP20)
Corresponds to the Muggle cliché “hold your horses!” – said to someone who’s trying to rush the speaker ahead of his or her story.

“losing a Knut and finding a Galleon” – Albus Dumbledore (OP27)
Albus Dumbledore said this to Minister for Magic Fudge who came to expel Harry from Hogwarts and instead apparently found evidence to convict Dumbledore of trying to overthrow him (OP27).

“lousy, biased scumbag” – Ron (GF20)
Ron, about Igor Karkaroff’s low score for Harry’s performance on the first task (GF20).

“Merlin’s Beard!” – Amos Diggory (GF6, GF25), Arthur Weasley (OP9), Dumbledore (OP27), and others
Merlin’s Beard is an exclamation of surprise, in reference to the famous magician Merlin. It is the most used wizard slang phrase used in the books, play and film scripts (18 times and counting).

“might as well be hanged for a dragon as an egg” (OP2)
Synonymous with the Muggle cliché “might as well be hanged for a sheep as well as a lamb“, meaning that if one is going to be punished severely for a lesser offense, one might as well go further and commit a more serious offense.

“Mudblood” (CS7)
Mudblood literally means “dirty blood”, a name for a witch or wizard who is Muggle-born.

“in the name of Merlin” – Ron (OP17)
Used to indicate emphasis.

“mule” (PS15, OP)
Calling a centaur by this is a grave insult.
Note: a mule is a horse/donkey cross, so it could be considered to be half-horse, analogous to a centaur; however, mules are used as beasts of burden by humans.

“nag” (OP30)
After the centaurs’ confrontation with him over Grawp, Hagrid referred to them as ‘ruddy old nags’ (OP30) after they’d left the scene (however, he’d called them mules to their faces, so he wasn’t chicken).
A nice double meaning here; a ‘nag’ can be a person who has been nagging someone else, but in another sense a ‘nag’ is a broken-down, worn-out horse, so there’s a suggested beast of burden connotation to Hagrid’s use of it here.

“obsolete dingbat” – Rita Skeeter (GF18)
Rita Skeeter called Albus Dumbledore this in an article for the Daily Prophet; he said the article was ‘enchantingly nasty’ (GF18).

“sent to the Centaur Office” (FB)
A Ministry of Magic in-joke, meaning the person in question is about to be sacked; since no centaur has ever used the Centaur Office, it must be very, very quiet in there. It is analogous to “taking the Chiltern Hundreds” – a legal fiction allowing sitting Members of Parliament in the House of Commons to resign (Wikipedia).

“wasn’t room to swing a kneazle” – Hagrid (OP20)
Corresponds to the Muggle cliché “not enough room to swing a cat” (kneazles are so cat-like that the two species can interbreed); meaning “very cramped”. It’s worth noting that Hagrid, like JKR herself, is allergic to cats (PS6).

“miserable old bat” – Ron (GF13)
Ron again, this time talking about Professor Trelawney after she assigned a ton of homework (GF13). This phrase is also used in the Muggle world.

“Squib”
Squib is a slang term for someone born into a wizarding family but having no magical ability.

“a tale worthy of Harry Potter” (OP4)
Expression coined by the Daily Prophet during their 1995-96 smear campaign, used to refer to any far-fetched story.

“stupid puffed-up, power crazy old…”
Hermione called Professor Umbridge this, although she was interrupted by Draco Malfoy and other members of the Inquisitorial Squad and never finished what she was going to say (OP28)

“Umbridge-itis“(OP30)
After the Weasley twins’ spectacular departure, droves of Umbridge’s Defence Against the Dark Arts students turned out to have been Skiving Snackbox customers, and would use them as soon as she appeared in class, claiming to be suffering from ‘Umbridge-itis‘ regardless of the specific symptoms they’d developed.

“working like house-elves” – Ron (much to Hermione’s chagrin) (GF14)
Hermione takes offense at Ron’s casual reference to what she sees as a gross injustice. Ron is comparing how hard he and Harry have been working to how hard the house-elves of Hogwarts work to maintain the castle. (There is an old-fashioned Muggle cliché that corresponds one-to-one with this, but it’s offensive.)

rude hand gestures
The portraits in Dumbledore’s office hiss and make rude hand gestures at Cornelius Fudge after they learn that he made a deal with Willy Widdershins for information and then ousted Dumbledore (OP2)