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Old English þeoh, þeh, from Proto-Germanic *theukhom (cf. Old Frisian thiach, Old Dutch thio, Dutch dij, Old Norse þjo, Old High German dioh), from PIE *teuk- from root *teu- "to swell" (cf. Lithuanian taukas, Old Church Slavonic tuku, Russian tuku "fat of animals;" Lithuanian tukti "to become fat;" Greek tylos "callus, lump," tymbos "burial mound, grave, tomb;" Old Irish ton "rump;" Latin tumere "to swell," tumulus "raised heap of earth," tumor "a swelling;" Middle Irish tomm "a small hill," Welsh tom "mound"). Thus thigh is literally "the thick or fat part of the leg."
The part of the leg between the hip and the knee. Also called femur.