'averse' originates from Latin, specifically the word 'aversus', where the prefix 'ab-' (often assimilated to 'a-') meant 'away' and 'vertere' meant 'to turn'.

'averse' changed from Latin 'aversus' into Late Latin/Old French forms (cf. Old French 'averse') and entered Middle English as 'averse', eventually giving the adjective 'averse' and its adverbial form 'aversely'.

Initially it meant 'turned away' (literally), but over time it evolved into the modern sense of 'having a strong dislike or opposition', with the adverb meaning 'in a reluctant or opposed manner'.