'appair' originates from Old French, specifically the verb 'apairer' (also spelled 'apareir' in some sources), where the prefix 'a-' functioned as a causative/denoting particle and the root related to making ready or fitting.

'appair' entered Middle English from Old French 'apairer' (Middle French forms such as 'apareiller' are related), appearing in early English texts as 'appairen' or similar spellings; over time its use declined and it became obsolete in modern standard English.

Initially it meant 'to make ready or fit' (prepare, fit out); over time the sense narrowed/shifted toward 'to pair or match' in some usages, and eventually the word fell out of common use, leaving related senses in modern derivatives (e.g. 'pair', 'prepare').