The short, thick first digit of the human hand, set apart from the other four fingers.
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Proto-Germanicbefore 900 CEwell-attested
From OldEnglish 'þūma' (thumb), from Proto-Germanic *þūmô (thumb), from PIE *tūm- / *tumH- (to swell, to be thick, to be puffed up). The thumb was named for being the thick, swollen digit — the one that sticks up and out. The same PIE rootproduced Latin 'tumēre' (to swell), from which English derives 'tumor' (a swelling), 'tumid' (swollen,
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'Thumb' and 'tumor' arethesame word. Both derive from PIE *tūm- (to swell, to be thick) — the thumb was 'the swollen one' and a tumor is 'a swelling.' The word 'thimble' also belongs to this family: OldEnglish 'þȳmel' (thumb-covering), from 'þūma' + the instrument