An expression of encouragement, support, or approval toward a boy, man, or male animal, used as a standalone interjection before or after a sentence. Largely presumed to be a corruption of the expression "that's the boy"; it is also sometimes spelled "atta boy." You won the spelling bee? Attaboy!
exclam. That is the way to do it! (Said to a boy or man.) Come on, Chuck. That-a-boy!