to throw up, usually after drinking

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

verb. Meaning to throw up or vomit.

Abbreviation of Cognac - i.e. expensive Brandy. Alcoholic beverage

cocaine, blow, powder, snow, gack

To be ridiculously wet shooting the basketball; used in same context as "swish" or "splash." Can also be used in other context, such as a long killstreak in Modern Warfare 2, and combined with other words to form fantastic contractions.

when a bitch ass nigga throws up