Attaboy

What does Attaboy mean and when to use it?

In the world of work, it means, "Good job!" Or, "Way to go!" The state police, one of the agencies where I worked, gave out "Attaboy Awards." Now, with the addition of a few women troopers, they probably have Attagirl Awards."

ATTABOY -- Military and police. "a commendation. 1975 Wambaugh 'Choirboys,' 288 'There's a lot of vice sergeants in this town that'd...write you an attaboy.'..." From Random House Historical Dictionary of American Slang, Volume 1, A-G by J.E. Lighter, Random House, New York, 1994.

I don't know what it stands for. That's my boy? Does anyone know?

The American Heritage Dictionary maintains that attaboy does indeed derive directly from "that's the boy!", which I presume was a once popular shout of encouragement or approval.

attaboy! exclam. = That's the boy! An expression of approval for deserving behavior or a successful performance. Also "attagirl!" Since c1910.
that's the (my) boy Fig., an expression of encouragement or pride. As of a father pointing with pride and saying, "That's my boy who did that."
From _Dictionary of American Slang_ by H. Wentworth & S.T. Flexner.

attaboy!/attagirl!/attababy! excl. [20C] (orig. US) a general excl. of admiration and encouragement. [? phr. 'that's the boy' etc or 'at her, boy' (E[ric]P[artridge]), where 'her' is neuter. Note US milit./police jargon _attaboy_, a commendation]