'externally-reinforced' originates from modern English, specifically a compound of 'externally' (from 'external') and 'reinforced' (from 'reinforce'), where 'external' derives from Latin 'externus' meaning 'outside' and 'reinforce' is formed from the prefix 're-' (again) + 'force' (strength).

'external' entered English via Old French/Medieval Latin from Latin 'externus', while 'reinforce' came into English from Old French constructions (e.g. 're-' + 'enforcir'/'forcier') and Middle English usage; the compound form developed in modern English to describe something strengthened by outside means.

Initially, 'external' meant 'of or relating to the outside' and 'reinforce' meant 'to make stronger (often again)'; combined, the phrase evolved to mean 'made stronger by outside support' with little change in the constituent senses.