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']
(..) Right, then! Smoke 'em if you got 'em. Have you used this? I used it to make some hot-air popcorn. That's about it. Okay! Let's make it happen, cap'n! Perfect. - You're back. - No. - How'd you know his head grows back? - It grows back? (..)2002 Men in Black II