Straight from Wikipedia, "Badonkadonk is a slang term for a woman's buttocks that are voluptuously large yet firm..." Per Tracy Morgan, "One cheek goes up, badonkadonk; the other goes down, badonkadonk." See also Ass, Body.
Per Missy Elliot, "Take my thong off and my ass go boom." See also Ass, Sex.
According to dictionary.com, this fancy sounding word means "having beautifully proportioned buttocks," as in "the quest for the callipygian ideal." See also Ass.