'ninety-one' originates from Old English, specifically the words 'nigontig' (ninety) and 'ān' (one), where 'nigontig' meant 'nine tens' and 'ān' meant 'one.'

'nigontig ān' transformed into the Middle English word 'nineti oon,' and eventually became the modern English word 'ninety-one'.

Initially, it meant 'nine tens and one,' and this meaning has largely remained the same in modern usage.