膝 means 'knee'

Characters for body parts often feature 月 radicals. That's true here, but these four kanji have much more in common. First, limbs and joints are prone to soreness, injury, and disability, requiring various healing modalities. Second, the shoulders, elbows, and knees can show how we're relating to other people; the Japanese associate these body parts with lending support, sitting close for an intimate talk, and reining in someone's behavior. Those body parts also play roles in martial arts and sumo. Finally, one's stance can express seriousness, tension, pride, shame, discouragement, respect, remorse, or a lack of reservedness.

The longer you live, the more shame you suffer, says an old Japanese proverb. Shame can involve feeling exposed. The Japanese experience deep embarrassment about misusing words (and kanji!). The exposure might be literal; essays 1572 and 2031 mention how evacuees feel when fleeing a disaster naked. Indeed, the body provokes much shame; essay 1572 features a shocking quiz with恥 (shame) in five genital-related words. Shame has its own body language. Essay 1212 on 肩 presents a common term related to shoulder position and shame, and essay 2100 discusses shame-filled, head-on-the-floor prostration when apologizing for a grave error.