'atta' originates from colloquial English, specifically as a clipped form of the spoken phrase 'that's a' (originally 'that is a'), where 'that' meant 'that' and 'is' meant 'is'.

'atta' changed from the informal exclamation 'attaboy' (early 20th century), itself derived from phrases like 'that's the boy' or 'that's a boy', and eventually became the standalone encouraging exclamation 'atta'.

Initially it indicated identification ('that is a [boy]'), but over time it evolved into its current exclamatory meaning of 'well done' or 'good job'.