腿
もも · momo · JLPT N1 · noun
femur
もも sounds like 'momo', the peach: a thigh is the big fleshy fruit of the leg.
The Japanese word もも (momo) is the same as the word for peach. Think of the thigh as the peachy, round fleshy part of your leg, the largest meaty part above the knee.
腿 — flesh radical 月 (肉) + 退 tuì (phonetic)
腿 is a semantic-phonetic compound. The left side 月 is the 'flesh/body' radical, marking it as a body part. The right side 退 supplies the sound (tuì → tuǐ). It's a relatively late character; older Chinese used 股 and 腳 for leg-related meanings.