At a boy

Any suggestion as to the origin of the idiom 'At a boy'?

Actually, this should be "Atta boy", a slangy, mutated form of "that's the boy"
Sort of like, "you da man" (or "you go, girl")

Merriam-Webster has the following entry:
''attaboy - (probably alteration of ''that's the boy'') - used to express encouragement, approval, or admiration''.