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']
(..) Power laces. All right. This thing doesn't fit. JACKET: DOC: Pull out your pants pockets. All kids here wear their pants inside out. Put on this cap. Perfect. You're the spitting image of your future son. MARTY: What? DOC: Help me move Jennifer over here. MARTY: So what's the deal? (..)1989 Back to the Future Part II