'unreinforced' originates from English, formed by the negative prefix 'un-' plus the past participle 'reinforced'; 'un-' meant 'not', and 'reinforce' comes via Middle French 'renforcer' (from Old French elements related to 'forcer') ultimately tied to Latin roots meaning 'strong'.

'reinforce' entered English from Middle French 'renforcer' (Old French 'enforcer'), became the English verb 'reinforce' in Early Modern English, then the past participle 'reinforced' combined with the productive English prefix 'un-' to form 'unreinforced' in modern English.

Initially it meant simply 'not strengthened' (i.e., lacking reinforcement); that basic sense has remained largely unchanged into modern usage, though the term is especially used in technical/engineering contexts.