肝腎
かんじん · kanjin · JLPT N1 · na-adj
essential
肝 (liver) + 腎 (kidney): two organs seen as vital, so it means essential or crucial
Traditional Chinese medicine treated the liver and kidneys as vital organs, so the pair came to mean essential or crucial. Kan is liver (肝) and jin is kidney (腎).
肝 — flesh radical 月 (肉) + 干 gān phonetic
A semantic-phonetic compound. The left side 月 is the 'flesh/meat' radical (from 肉), used for body organs. The right side 干 supplies the sound 'gān'. Together they denote the liver, an internal organ.