The definition of an atta girl is a compliment or words of encouragement given to a girl who has done a good job on something.
(noun)An example of an attagirl is when your boss tells everyone at the meeting that you did a great job on your last project.
See attagirl in Webster's New World College Dictionary
interjection
See attagirl in American Heritage Dictionary 4
interjection
Origin:
Origin: Alteration of That's the girl!
.