Draco dormiens nunquam titillandus

Notes

The image of the school crest first appeared on the title page of the Bloomsbury editions of the novels and on the 1998 Bloomsbury "Harry Potter Fan Club" material written by Rowling.

Rowling discussed the origin of the motto in interviews:

How did you think up the motto "Never tickle sleeping dragons", which appears under the crest? Is there a story about it?

JK Rowling: You know the way that most school slogans are thing like persevere and nobility, charity and fidelity or something, it just amused me to give an entirely practical piece of advice for the Hogwarts school motto. Then a friend of mine who is a professor of classics - my Latin was not up to the job, I did not think it should be cod Latin, it is good enough for cod Latin spells, that is they used to be a mixture of Latin and other things. When it came to a proper Latin slogan for the school I wanted it to be right, I went to him and asked him to translate. I think he really enjoyed it, he rang me up and said, "I think I found the exactly right word, 'Titillandus'", that was how that was dreamt up.

And:

SF: And names like that. Even the school crest is something which is rather fun for those of us who have done a bit of Latin. For instance: “Draco dormiens nunquam titillandus” – it’s sort of like “Let sleeping dogs lie”, but it is “Don’t tickle a sleeping dragon”.
JKR: “Don’t tickle a sleeping dragon”, exactly.
SF: Which is fine advice.
JKR: I wanted good practical advice. All the schools I’ve ever been to or taught in have mottos: “Persevere”, “Onwards and Upwards”. I wanted good solid practical advice for Hogwarts.