From Old English þēoh, from PIE *tewk- 'to swell' — the thigh was named for being the thick part of the leg.
From Old English þēoh 'thigh,' from Proto-Germanic *þeuhą, from PIE *tewk- 'to swell, be fat.' The thigh was named as the thick, fleshy part of the leg. The word underwent significant sound changes but kept its meaning intact. Key roots: *tewk- (Proto-Indo-European: "to swell, be fat, be strong").
'Thigh' and 'thumb' share the same PIE root *tewk- 'to swell' — both were named for being the thick, swollen members of their respective limbs.