In less urban environments, the term is more commonly used to describe the act of vomiting.
noun: yack means congac
verb: yack means to vomit
I drank too much, I think I'm gonna yack.
I was yackin' on some bitches in M-Dub Deuces last night.
Combinations: yackadaisical, yack in the box, Yackensack, yackity-yack don't talk back, yack my bitch up, etc.