'mere' (adjective) originates from Old French, specifically the word 'mere' (from Latin 'merus'), where 'merus' meant 'pure, unmixed'. Also, 'mere' (noun) originates from Old English, specifically the word 'mere', where 'mere' meant 'lake, pool, sea'.

'mere' (adjective) passed into Middle English via Anglo-Norman/Old French from Latin 'merus' meaning 'pure' and developed the sense 'sheer, simple' in modern English. 'mere' (noun) continued from Old English 'mere' (from Proto-Germanic *mari) into Middle English unchanged and survives in modern English as an archaic or dialectal term for a lake or pool.

Initially, the root for the adjective sense meant 'pure, unmixed' and evolved into the sense 'sheer, mere, only' in modern usage. Initially, the noun sense meant 'sea, lake, pool' and this basic meaning has remained, though its use is now archaic or regional.