'retreater' originates from Middle English, specifically from forms of 'retreat' (such as 'retreten'), which ultimately derive from Latin 'retrahere', where 're-' meant 'back' and 'trahere' meant 'to draw or pull.'

'retreater' changed from Middle English forms like 'retreten' and Old French terms related to 'retrait'/'retret', and the modern English noun was formed by adding the agentive suffix '-er' to 'retreat'.

Initially it referred to the act of drawing back or withdrawal ('retreat' as an action), but over time it also came to denote a person who withdraws or a person who attends a retreat; the noun sense 'one who retreats' is now established.