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']
(..) Mm. Uh-- Classic Rock is clear. I'm gonna go check out Punk. And if I find anybody in there... I'm gonna pop a fuckin' cap in their ass. I'm gonna call it self-defense. Hello? I'm on my own. Oh, God. (..)2006 Tenacious D in the Pick of Destiny