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']
(..) (WHISTLE BLOWS) You could look like me, though. If you dressed like me and shaved your hair off. I'm not shaving my hair off. - You could cover it with a cap. - My pyjamas are a different colour. I could bring some. There's a hut full of them, thousands of them. Would you do it, though? (..)2008 The Boy in the Striped Pajamas