'thumb' originates from Old English, specifically the word 'þūma', where Proto-Germanic '*þūmô' meant 'thumb'.

'thumb' changed from the Old English word 'þūma' and through Middle English forms such as 'thum'/'thoum' eventually became the modern English word 'thumb'.

Initially, it meant 'the thumb (digit)', but over time it retained that primary meaning while also developing extended senses (for example, 'rule of thumb' meaning an approximate guideline).