thumb: [OE] The thumb is etymologically the ‘swollen’ part – an allusion to its greater thickness than the other fingers. Along with its relatives German daumen and Dutch duim, it goes back to a prehistoric West Germanic *thūmon. This in turn can be traced to Indo- European *tum- ‘swell’, which also produced English tumour and tumult. The b in thumb appeared in the early Middle English period, when it was still a two-syllable word (thumbe), and at first was pronounced, but it has fallen silent over the centuries. => thigh, thimble, tumour, tumult
