He is calling her captain, it's slang

cap would generally be considered the slang version of captain like paige said. i'm american and to my knowledge we don't typically use it in that way outside of maybe the navy where captain is a rank but also can be a general title for the commanding officer, whether they are actually the rank of captain or not.

It's unclear why he calls her that. The context in what you are reading would make that more clear.