In lunfardo is mafia boss, important person. By extension he is someone superior, the leader in his occupation, the chief in any task even if it is not unlawful as in his original sense. It comes from the Italian capo ("head" ). See foreman, capable, capanga .
It's a little captain's form. It means leader, boss, boss, head, warlord, guide. Although the term has been degenerated in general, by relating it to drug trafficking and crime, the truth is that it can and should be used in the accepted sense.
Capo : Insect that lives in fruits.