膝 is rarely used alone in modern Chinese. The common word for knee is 膝盖 (xīgài). 膝 appears in literary or compound words: 促膝谈心 (chùxītánxīn, to have a heart-to-heart talk, literally 'touching knees and talking').
memory
The radical 月 (flesh/body) on the left marks it as a body part. The right side 桼 gives the approximate sound xī.
