'twentieth' originates from Old English, specifically the word 'twentigotha', where 'twentig' meant 'twenty' and '-otha' was a suffix for ordinal numbers.

'twentigotha' transformed into the Middle English word 'twentithe', and eventually became the modern English word 'twentieth'.

Initially, it meant 'the ordinal form of twenty', and this meaning has largely remained the same in modern usage.