These are the 3 Slang meanings I know of:
Donk - A late seventies or early eighties sedan, modified to accommodate bigger wheel size. DONK from the originals made from Chevy Impalas - mistakingly the impala became a donkey - donk for short
given by Punkic
Donk - A girls large backside - from be-donk-a-donk (or similar) most likely referring to the "donk" beat present on many R&B, Funk, and rap tracks. to which a girl might shake her "moneymaker"
also given by punkic
Donk - A poker player who refuses to fold a bad hand. Donk short from Donkey - Stubborn
Tats suggestion
Given the context I don't know what was meant by it: could be any of these or the suggest of N.G.H