'long-acting' originates from Old English and Latin elements, specifically the Old English word 'lang' (later 'long') and the Latin word 'actus' (via English 'act'), where 'lang' meant 'long' and 'actus' meant 'a doing or action'.

'long-acting' was formed as a modern English compound combining Old English 'lang' (which evolved into 'long') with the present participle 'acting' (from Middle English 'act' < Old French/Latin 'actus'), and it eventually became the hyphenated modern English adjective 'long-acting'.

Initially built from roots meaning 'long' + 'acting' (i.e., 'acting for a long time'), its usage narrowed over time to specifically describe something—especially a medication or treatment—that produces effects lasting for an extended period.