'accustom' originates from Old French, specifically the word 'acostumer,' where 'a-' meant 'to' and 'costume' meant 'custom or habit.'

'acostumer' transformed into the Middle English word 'accustumen,' and eventually became the modern English word 'accustom'.

Initially, it meant 'to make familiar with a custom or habit,' and this meaning has largely remained the same in modern usage.