"Don't worry, the Weasleys are more than a match for the Bludgers -- I mean, they're like a pair of human Bludgers themselves."
-- Oliver Wood about Fred and George Weasley (PS10)
A Bludger is a round jet-black iron ball, 10 inches in diameter – that is, slightly smaller than a Quaffle – one of the four balls used in playing the game of Quidditch (in which two Bludgers are used at any one time). A Bludger always attacks and attempts to unseat the nearest player, which is why Beaters attempt to knock Bludgers toward opposing players (PS10, QA6).
Originally a Bludger was just a rock enchanted to make it attack the players as a modern Bludger does, but the use of rocks was abandoned in favour of other substances since Beaters’ bats could eventually smash them into gravel (QA6).
Lead was experimented with, but abandoned because after a lead Bludger had taken a beating, indentations would be left on its surface that affected its ability to fly properly (QA6).
An early name for a Bludger was “Blooder“, referring to the likely effect of being hit by one (QA6).
The Banchory Bangers team was disbanded for allowing their Bludgers to escape after a particularly wild post-match party (QA5).
A Bludger is mentioned in a Norwegian poem from the 1400s written by Ingolfr the Iambic:
“As I draw ever closer, the crowd gives a shout
But then comes a Bludger and I am knocked out.” (QA8)
Bludger, as the word is used in Quidditch, may have derived from the English word bludgeon, meaning a "thick stick with a heavy end, used as a weapon" or "to beat (someone) repeatedly with a bludgeon or other heavy object" (Oxford English Dictionary). Alternatively, in Australian slang, a bludger is a lazy or idle person who avoids work (Oxford English Dictionary).
Lexicon timeline of Quidditch
Lexicon list of Quidditch equipment and accessories
From the Web
WizardingWorld.com (Pottermore) features:
Harry Potter Wiki: Quidditch