Bludger Backbeat is a method of hitting the Bludger backwards, difficult to pull off with any precision but very effective for disorienting opponents, who don’t expect a Bludger to come at them in that way (QA10).
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).
Although a backbeat is being used in Quidditch to illustrate that the Beaters are beating (hitting) the Bludger backwards, it is also a musical term denoting a strong accent on an unaccented beat of the bar (Oxford English Dictionary).
Lexicon list of Quidditch equipment and accessories