The car was filthy when he returned it to me, and 'to cap it off' [='to top it off'], there was almost no fuel left in the tank.
a 'shower/bathing cap' [=a head covering that people wear to keep their hair dry when they are showering/swimming]
They may not like being called careless, but 'if the cap fits, wear it'. [=('US') 'if the shoe fits, wear it']
(..) I never gave much thought to what it meant. I just thought it was some cold-blooded shit... to say to a motherfucker before I popped a cap in his ass. But I saw some shit this mornin' made me think twice. See, now I'm thinkin'... maybe it means you're the evil man... (..)1994 Pulp Fiction
(..) I'm prepared to scour the earth for that motherfucker. If Butch goes to Indochina... I want a nigger hidin' in a bowl of rice... ready to pop a cap in his ass. I will take care of it. Mister? ESMARELDA: Hey, mister? What? (..)1994 Pulp Fiction