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.