'auxiliary' originates from Latin, specifically the word 'auxilium', where the root 'aux-' meant 'help' and the suffix '-arius' indicated 'pertaining to'.

'auxilium' (Latin) developed into Late Latin 'auxiliarius' and Old French 'auxiliaire', then entered Middle English as 'auxiliarie' and eventually became the modern English word 'auxiliary'.

Initially it meant 'help or aid' (from Latin 'auxilium'), and over time it came to mean 'providing supplementary help; a helper; a supporting force' and also took on the grammatical sense 'helping verb'.