Yet another slang term for the female genitalia; the word is somewhat milder than the worst word in the English language, but equal to or worse than some other words.
It grates on your ear, doesn't it? You probably could've guessed its meaning before I even defined it.
Maybe it's the "sn" at the beginning (reminiscent of words like snatch) or the "izz" at the end (reminscent of other words). Whatever the reason, it conjures up images of some short, hairy, sleazy guy with gold chains who spends most of his time in bars and locker-rooms, loudly relating fictitious and degrading stories about his putative "accomplishments" with women.
The term is relatively rare in the United States, but may be more common in the United Kingdom; it achieved its greatest currency during the publication of Shaved Snizz, a short-lived men's magazine put out by the publishers of Genesis.