'auxilium' originates from Latin, specifically the word 'auxilium', where the root 'aux-' (related to 'augēre') meant 'to increase, to help or promote'.

'auxilium' passed through Classical and Medieval Latin; related adjectival and verbal forms (for example 'auxiliarius' and 'auxilio') influenced Old French and Anglo-Latin vocabulary and eventually contributed to English derivatives such as 'auxiliary' and 'auxilium' as a learned borrowing in specific contexts.

Initially, it meant 'help' or 'that which increases/supports'; over time its core sense of 'aid or assistance' has largely remained in both specialized (military, legal, ecclesiastical) and general uses, while derivatives expanded into the adjective 'auxiliary' meaning 'providing supplementary help'.