What’s the Meaning of Atta Boy?
Attaboy is short for “that a boy” it just means, “there ya go” or “that’s the spirit!” It’s more a response than a statement by itself. It is used to cheer on somebody and to express enthusiastic encouragement or approval to a boy.
If someone would say, “After careful consideration, I think I’ll go anyway” and the response would be: “atta boy!” It’s an exclamation which is sort of old hat. That is the reason you will see it in literature and old movies. It’s not really used anymore.