in referance the Lil Jon & The East Side Boyz song "Da Blow" where the hook states "the blow, the pills, the yack, the herb". The term "yack" is actually referring to cognac (pronounced: "cone-yack"). So in essence the song is referring to the blow (cocaine), the pills (ecstacy and/or pharmacutical), the yack (cognac), the herb (marijuana). Which covers all the most common methods of getting fucked up.
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
Da blow, da pills, da yack, da herb... (repeat)
I drank too much, I think I'm gonna yack.