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

'thirty' changed from the Old English word 'þrītig' (from Proto-Germanic '*þrīzti') and 'one' from Old English 'ān'; these elements were combined in modern English to form the numeral 'thirty-one'.

Initially, it meant 'three tens plus one', and over time this numeric meaning has remained essentially unchanged.