'bargainor' originates from English, formed by adding the agentive suffix '-or' to the noun 'bargain', which itself comes from Old French 'bargaigner' meaning 'to haggle' or 'to negotiate'.

'bargainor' developed from Middle English agentive forms such as 'bargaynour' or 'bargayner' and the related Modern English form 'bargainer'; the suffix '-or' is a Latin-derived agentive ending used in English formation.

Initially it meant 'one who makes or offers bargains' in ordinary speech; over time the form became less common in general usage and is now largely archaic or specialized (legal) for 'a party to a bargain'.