'twenty-something' originates from English, specifically combining the numeral 'twenty' and the word 'something', where 'twenty' meant '20' and 'something' meant 'an unspecified thing or person'.

'something' comes from Old English elements such as 'sum' (some) + 'þing' (thing), and 'twenty' comes from Old English 'twentig'; these elements combined in modern English usage to form the compound 'twenty-something' to refer to an unspecified person in their twenties.

Initially it could be interpreted as 'twenty and some (amount)', but over time it evolved into the specific sense of 'a person who is in their twenties' (and the plural 'twenty-somethings' for a group).