'exactly' originates from Latin, specifically the word 'exactus', where 'ex-' meant 'out' and the root related to 'agere'/'exigere' meant 'to drive, to demand'. The adverb was formed in English by adding the suffix '-ly' to 'exact'.

'exactly' changed from Latin 'exactus' to Old French/Middle English 'exact' and the adverbial suffix '-ly' was added in Late Middle English to form the modern adverb 'exactly'.

Initially related to meanings like 'driven out' or 'demanded' and later 'carefully measured' or 'precise'; it evolved into the modern senses of 'precisely' (adverb) and an emphatic agreement (interjection).