'attaboy' originates from colloquial American English, specifically from the phrase 'that's a boy', where the spoken contraction 'that's a' became 'atta' and 'boy' meant 'male child' (later generalized as a term of approval).

'attaboy' changed from the colloquial two-word exclamation 'atta boy' used in early 20th-century American English and eventually became the single written form 'attaboy'.

Initially, it meant 'that's a boy' (praising or calling attention to a male child), but over time it evolved into its current meaning of 'an exclamation or expression of approval or encouragement' used for any person.