'thirty-one' originates from Old English, specifically the words 'þrītig' (thirty) and 'ān' (one), where 'þrī' meant 'three' and 'ān' meant 'one'.

'thirty-one' changed from the Old English compound of 'þrītig' + 'ān' through Middle English forms (e.g. 'thritti-ane'/'thirty ane') and eventually became the modern English 'thirty-one'.

Initially it meant a count of 'three tens plus one' and over time this concrete numerical meaning has remained essentially the same as the modern meaning 'thirty-one'.