To "Seatbelt it up" is to tuck a rather awkward boner up into the elastic of your jocks/briefs as to hide it from the general public.
Dave - Oh man I just got a hard on from that girl, what should I do??
Nick - Relax mate, just Seatbelt it up and away you go!
Nick - Relax mate, just Seatbelt it up and away you go!
by C-Man-AU March 9, 2011