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

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

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