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']
(..) An ultra-secret organization dedicated to fighting crime on a global scale. Rob a bank, we're not interested. Kill someone, not our deal. But you want to melt the polar ice caps, or vaporize Mount Fuji, or even steal the Moon... Then we notice. First of all, you got no proof that I did that. (..)2013 Despicable Me 2