'eighteenth' originates from Old English, specifically the word 'eahtatēoþa,' where 'eahta' meant 'eight' and '-tēoþa' meant 'tenth.'

'eahtatēoþa' transformed into the Middle English word 'eighteenthe,' and eventually became the modern English word 'eighteenth.'

Initially, it meant 'the ordinal number following the seventeenth,' and this meaning has largely remained the same in modern usage.