'unarched' originates from the negative prefix 'un-' (Old English), meaning 'not', combined with 'arched', formed from English 'arch' (ultimately from Latin 'arcus'), where 'arcus' meant 'bow' or 'arch'.

'arch' comes from Latin 'arcus' → Old French/Anglo-French 'arc'/'arche' → Middle English 'arche'/'arch', and the past-participial/adjectival form 'arched' developed in English; the prefix 'un-' (Old English) was later attached to form 'unarched'.

Initially it simply meant 'not arched' (lacking an arch); this basic negative/adjectival meaning has largely been retained in modern usage.