'unbeliever' originates from English, specifically the combination of the prefix 'un-' (from Old English 'un-') meaning 'not' and the noun 'believer' meaning 'one who believes'.

'unbeliever' was formed in Middle English by attaching the negative prefix 'un-' to the agent noun 'believer' (itself derived from the verb 'believe', from Old English forms such as 'belyfan'/'belȳfan'); the compound stabilized into the Modern English word 'unbeliever'.

Initially it meant 'a person who is not a believer', and over time this basic meaning has remained largely unchanged (though usage has broadened to include nonreligious contexts, e.g., unbelievers of a claim).