'19th' originates from the Old English word 'nigontēoþa', where 'nigon' meant 'nine' and 'tēoþa' meant 'tenth'.

'Nigontēoþa' transformed into the Middle English word 'nigontenthe', and eventually became the modern English word 'nineteenth'.

Initially, it meant 'the ninth after ten', and this meaning has largely remained the same in modern usage.