Origin of attaboyprobably from that's the boy!
Origin of attaboyAlteration of That's the boy!
- (idiomatic, colloquial) Used to show encouragement or approval to a boy or man.
- Attaboy! That's the way to hit a home run!
- A cry of "attaboy"; an accolade.
The OED gives 'attaboy' as a "careless pronunciation" of "that's the boy!", though the more common expression is "that's a [good] boy!", which derives from the expressions "That's my [good] boy" which first appears in Shakespeare's Titus Andronicus.
It has otherwise been suggested that attaboy goes back to the hunters’ language, in which boy acquired the meaning “hunted animal.” attaboy (ataboy, as it is sometimes spelled, because no one knows the “right” spelling of this phrase) would go back to "a tout a boy!" ("a tout", a call to incite dogs). (Attaboy! Or, The Male-Intimate Affectionate Overtones Questions By Anatoly Liberman, December 13th, 2006)