WordReference Random House Unabridged Dictionary of American English © 2017
at•ta•boy  (atə boi′),USA pronunciation interj. [Informal.]
  1. (used as an enthusiastic expression of encouragement or approval to a boy, man, or male animal.)
  • alleged to be a reduced form of that's a boy or that's the boy 1905–10, American.

'attaboy' also found in these entries:

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