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']
(..) I'll take care of it. Hey, what's all that? The high-roller package. It's what you guys order. I have coffee mugs. You have baseball caps, huh? And fancy calendars all with pictures of Stu and Jade. Her name's Jade?/ Yeah, she's beautiful, man. Clean. Very tight. Tits, like that. (..)2009 The Hangover