The word thumb comes from the Middle English thoume, thoumbe, from Old English thuma; akin to Old Low German thumo, Old High German dumo, Latin tumere to swell. 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 thûmon. 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. The words thigh and thimble have related origins. It’s mere coincidence that thumb rhymes with plumb, to think about or examine in depth.