'fast-acting' is a compound formed from the adjective 'fast' and the present participle 'acting'. 'fast' originates from Old English, specifically the word 'fæst', where 'fæst' meant 'firm' or 'fixed'. 'act' (in 'acting') ultimately comes from Latin 'agere' meaning 'to do' or 'to drive'.

'fast' originally meant 'firm, fixed' in Old English ('fæst') and later developed senses including 'secure' and then (by modern English) 'quick' in certain contexts; 'acting' comes from Latin 'agere' via Old French/Latin-derived forms and Middle English. The compound 'fast-acting' arose in modern English (notably in the 19th–20th century) to describe remedies and processes that work quickly.

Initially, 'fast' meant 'firm' or 'fixed', but over time one sense evolved to mean 'quick'. Consequently, the compound 'fast-acting' came to mean 'producing rapid effect' in modern usage.