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.
Base from an acronym used in computer terms meaning "Zero Insertion Force" Also refers to a Loose whoe or skank and the defining fact being that it takes Zero Force for Insertion.
Shortened form of as-if, used by those who find 'sif' to be a travesty of english. I mean really, " 'sif " would be alright, but sif, nah, while your killing the language, at least make it Phonetic.