'autrefois' originates from French, specifically the phrase 'autre fois' (modern French 'autrefois'), where 'autre' meant 'other' and 'fois' meant 'time/occasion'.

'autrefois' comes from Old French 'autre fois' (a compound of 'autre' + 'fois'); 'autre' itself derives from Latin 'alter' meaning 'other', and 'fois' ultimately traces back to Latin 'vicis' (via Vulgar Latin), meaning 'turn' or 'change' (hence 'time' or 'occasion'). The phrase was borrowed into English usage (literary contexts) with its meaning intact.

Initially it meant 'at another time' or 'on another occasion'; over time it became used as an adverb meaning 'formerly' or 'in former times' in both French and borrowings into English.