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']
(..) GURGLING NOISES] PAUL: Hush up there, Toot. TOOT-TOOT: Getting screwed. I'm getting the cap. BRUTAL: Arlen Bitterbuck... ...electricity shall be passed through your body until you are dead... ...in accordance with state law. (..)1999 The Green Mile
(..) Let's keep going. Harry. Soak the sponge. TOOT-TOOT: Putting on the black hood. And putting on my cap. Why's he putting that sponge on his head? Conducts electricity to the brain fast like a bullet. You don't ever throw the switch (..)1999 The Green Mile