Several parts combine into one character.

膝 is a phono-semantic compound: 月 (the flesh radical, marking it as a body part) + 桼 (qī, "lacquer tree" — phonetic). 桼 lent the sound; 月 carried the meaning. Composite reading: "the rounded fleshy joint of the leg = knee." The flesh radical 月 (a stylized 肉) anchors the entire CJK body-part vocabulary.

Korean reading "seul." 膝下 (seul-ha, "below the knees" = at one's parents' side — a literary expression for being raised by one's parents and used in formal Korean to indicate one's family origin), 膝關節 (seul-gwanjeol, knee joint — medical). 膝下 still appears in dignified Korean prose as a formal way to refer to the place and parents that raised a person.

Mandarin xī, 1st tone. 膝 (xī), 膝盖 (xīgài, knee — daily), 跪膝 (guìxī, kneeling). 膝盖 dominates everyday Chinese — "I scraped my knee" in Mandarin is 我膝盖擦破了 (wǒ xīgài cā pò le), with 膝盖 ("knee-cap") rather than the bare 膝.

Japanese on-reading シツ (shitsu) — 膝関節 (shitsukansetsu, knee joint — medical). Kun-reading ひざ (hiza) — 膝 (hiza, knee), 膝枕 (hizamakura, "knee-pillow" = lying with one's head on someone's lap). 膝枕 is one of the most beloved Japanese pop-culture tropes — manga, anime, and live-action drama have built thousands of intimate scenes around the image, to the point where the word itself carries strong emotional and romantic weight.

Memory aid: flesh (月) plus the phonetic 桼 — the rounded fleshy hinge of the leg.