used to address someone, could be used by a close friend or an obnoxious person who thinks they are hilarious. similar to buddy boy, "pally boy" is just 'pal' (meaning 'friend') turned into an adjective to describe 'boy'.
Sara: wanna go to lunch?
Hiro: mmmm nah, I'm good.
Sara: alright, see you later then, pally boy.