1. A quick masturbation session, or quick sex. Or a statement that one is busy, but only for a short period of time. Can also be used to refer to a third party's where-abouts in a joking manner.
"Got caught having a quick biff this morning"
"She told me to meet her in the bathroom. A quick biff later and im back at my desk working."
"Whats up?" "Quick Biff" or "Brb, quick biff."
"Where did Ben go?" "I dunno...quick biff?" "hahaha"