'attagirl' (or 'atta girl') originates from colloquial English; 'atta' is a reduced form of the phrase 'that's a' (from 'that is a'), combined with 'girl'.

'atta' developed as a phonetic contraction of 'that's a' in dialectal spoken English; the combined exclamation 'atta boy'/'atta girl' became established in informal speech in the late 19th to early 20th century and has persisted as an idiomatic cheer.

Originally a clipped spoken form of the literal phrase 'that's a girl' (pointing out or identifying), it evolved into an interjection meaning 'well done' or 'good job' used to praise or encourage.