short for "As if!" ... used as an exclamation of doubt for the claims of another. Derogatry / superior - the utterer of this phrase does not trust the judgment of his colleague, it is a sign of disbeleif which edges towards disrespect.
"Never mind, I'll try harder next time"
"I'm the biggest beast in the jungle!"
"yeah right man, zif!"
"Tupac still lives man!"
"zif... you have to let go, man"
"I buy all me threads from Armani"
"zif - you looks like a chav from pahndstretcher"
"Honest governor I am not a 'omesexual, that was just an experiment"
"zif! you is a batty boy."