A colloquial term for masturbation, or slapping the salami.

Taking a shit in the bathroom

If you throw up Jack Daniels in a Spanish speaking country. In English you would Yack Jack, however Spanish people pronounce words starting with J as Y, hens Yack Yack.

to vomit after drinking heavily

to throw up, usually after drinking

verb. Meaning to throw up or vomit.

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