Literal translation into the American Language is "Captain". People have used and continue to use this term to characterize a Mafia family member to signify their relevance and importance to their family. A captain "orders" soldiers to do certain tasks needed. Capos get their orders from underbosses, who get them from the family Consigliere, who gets his from the boss.
What you say when you're extremely high (preferably on weed) and somebody says something that's totally fucking out there to the point where you (or the rest of the group) can't even understand it stoned.
Dude 1: Ah man, this song's in standard, and i'm in... whatever you call everything a step down.
Dude 2: No need to re-tune, just put a capo on the first fret!
Dude 1: but now the 12th fret nat. harmonic isnt on the 12th fret.
Dude 2: SHUT UP!