'efficacious' originates from Latin, specifically the word 'efficax', where the stem comes from Latin 'efficere' (from 'ex-' + 'facere'), meaning 'to bring about' or 'to accomplish'.

'efficacious' developed from Late Latin 'efficax' (and related post-classical Latin forms such as 'efficac-') and entered English in the early 17th century, ultimately becoming the modern English adjective 'efficacious'.

Initially it meant 'producing or able to produce the desired effect', and this core meaning has largely remained the same in modern usage.