Attaboy
Entry in progress—B.P.
(Oxford English Dictionary)
attaboy, int.
Etymology: Said to represent careless pronunciation of that’s the boy! (boy n.1 3b).
slang (chiefly U.S.).
An exclamation expressive of encouragement or admiration.
1909 Amer. Mag. May 40/2 Back of Chance’s war cries, ‘At-a-boy’, or ‘Now ye’re pitching’, may be hidden a whole command to his team.
1917 C. Mathewson Second Base Sloan xxii. 298 ‘Ata boy!’ called the Damascus catcher.