'threescore' originates from Old English, specifically the word 'þrīe scora', where 'þrīe' meant 'three' and 'scora' meant 'score' (a group of twenty).

'þrīe scora' transformed into the Middle English word 'threescor', and eventually became the modern English word 'threescore'.

Initially, it meant 'three times twenty', but over time it evolved into its current meaning of 'sixty'.