膝

Left-side flesh radical - the side form of 肉, visually identical to 月 moon but historically distinct. It tags 膝 as a body part, the standard prefix for anatomy chars alongside 脚 foot, 腿 leg, 胳 arm, 肩 shoulder. The radical-variant entry makes the meat-vs-moon distinction explicit when learners tap through.

Right side 桼 supplies the sound - qī drifting to xī through a regular palatalization shift. 桼 is an older form of 漆 lacquer, depicting sap dripping from a cut tree. The connection to knees is purely phonetic; meaning rides on the flesh radical alone.