'responder' originates from Latin, specifically the word 'respondere', where 're-' meant 'back' and 'spondere' meant 'to pledge/answer'.

'respondere' passed into Old French as 'respondrer/respondre' and into Middle English as forms like 'responden'/'responden'; the English agent noun 'responder' was later formed by adding the suffix '-er' to 'respond'.

Initially related to 'pledging' or 'answering' in Latin, the sense evolved into 'to reply' in modern English; the agent noun came to mean 'one who replies' and later extended to 'one who responds at emergencies' (as in 'first responders').