Attaboy

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early 20th century: probably representing a casual pronunciation of that's the boy .

wiktionary

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Variant pronunciation of "That's the boy!", a cry of encouragement, especially to children and pet dogs.

etymonline

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attaboy (interj.)

1909, originally in baseball slang, said to be from common pronunciation of "that's the boy!" a cheer of encouragement or approval. I'm the boy for ______ "I'm willing and capable at" is attested from 1843. Related: Attagirl (1924).