Bumphing is a Quidditch foul where the Beater intentionally hits a Bludger towards the crowd in order to halt the game momentarily, thereby denying an opposing Chaser a score (QA6).
This would have been one of the many fouls committed at the first Quidditch World Cup in 1473, where an instance of every known foul occurred (QA6).
Commentary
Etymology
bumph is an alternate spelling of the word bumf, which means "useless or tedious printed material" (Oxford English Dictionaries). People often feel that the bumph they receive in the post, with magazines or inside newspapers is an annoying distraction - a bit like hitting a Bludger into the crowd to stop the match.
From the Web
WizardingWorld.com (Pottermore) feature: Quidditch Through the Ages
Writing by J K Rowling on Imgur (Daily Prophet coverage of the 2014 Quidditch World Cup, originally from Pottermore): History of the Quidditch World Cup
Harry Potter Wiki: Bumphing
Pensieve (Comments)
Tags: brooms / broomsticks cheating crowds fairness fouls match sportsmanship tactics