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