Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio.
Informal indicating approval or encouragement: said to a man or boy
also sp. atta boy
Used by arrangement with John Wiley & Sons, Inc.
The American Heritage® Dictionary of the English Language, 4th edition Copyright © 2010 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Informal Used to show encouragement or approval to a boy or man: Attaboy! That's the way to hit a home run!
Origin: Alteration of That's the boy!.