That-a-boy!

attaboy

An expression of encouragement, support, or approval toward a boy, man, or male animal, used as a standalone interjection before or after a sentence. Largely presumed to be a corruption of the expression "that's the boy"; it is also sometimes spelled "atta boy." You won the spelling bee? Attaboy!
Farlex Dictionary of Idioms.

That-a-boy!

exclam. That is the way to do it! (Said to a boy or man.) Come on, Chuck. That-a-boy!
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions
See also:
  • ata
  • atta
  • attaboy
  • thatta
  • attagirl
  • gal
  • attagal
  • oh man
  • hear, hear
  • hear, hear!
References in periodicals archive
"That-a-boy!" Buxbaum cheers as Loyola hits a clutch 3-pointer with 6.2 seconds left to propel Loyola to a 69-68 victory and an Elite Eight matchup Saturday.