Definition of attaboy exclamation from the Oxford Advanced Learner's Dictionary

 

attaboy

 exclamation
exclamation
BrE BrE//ˈætəbɔɪ//
 
; NAmE NAmE//ˈætəbɔɪ//
 
(informal, especially North American English)
 
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used when you want to encourage somebody or show your admiration of them, especially a boy or man see also attagirl Word Origin early 20th cent.: probably representing a casual pronunciation of that's the boy.
See the Oxford Advanced American Dictionary entry: attaboy