Several parts combine into one character.

肝 is a phonetic-semantic compound: 月 (flesh) + 干 (gān, "shield" — providing the sound). What makes 肝 culturally rich is that traditional East Asian medicine assigned the liver an extraordinary metaphysical role: the liver was considered the body's shield against external attack, the seat of the spirit, and crucially the seat of courage and decisiveness. The very phonetic 干 ("shield") embeds this conception. From this medical-philosophical framework, 肝 took on a second meaning across all CJK languages: not just the organ, but bodily courage / nerve / mettle. The English idiom "to have guts" maps almost exactly onto the CJK use of liver-as-courage.

Korean reading "gan." 肝臟 (ganjang, liver — anatomical), 肝炎 (ganyeom, hepatitis), 肝硬變 (gangyeongbyeon, cirrhosis), and the literarily charged 肝膽相照 (gandam-sangjo, "liver and gallbladder reflecting each other" — a Tang-era idiom for sincere friendship where two people share their innermost feelings). Korean colloquial: ("the liver is big") = bold / brave; ("the liver is small") = cowardly / timid. The Korean compound (ganjang) homophonically also means "soy sauce" — same hangeul, different etymology.

Mandarin gān, 1st tone. 肝 (gān, liver), 肝脏 (gānzàng, liver — anatomical), 肝炎 (gānyán, hepatitis), 肝胆 (gāndǎn, "liver and gallbladder" = courage / heart-and-soul). And the affectionate 心肝 (xīngān, "heart-liver") used as a term of endearment for one's most beloved — child, lover, or grandchild: 心肝宝贝 (xīngān bǎobèi, "heart-liver treasure-baby") is what a Chinese grandmother might call her favorite grandchild. The "vital organs" become metaphors for what is most precious.

Japanese on-reading カン (kan) — 肝臓 (kanzō, liver — medical), 肝炎 (kan'en, hepatitis), 肝心 (kanjin, "liver-heart" = the most important / crucial — note the same metaphorical leap as Korean / Chinese, where vital organs = essential matters). Kun-reading きも (kimo) is wonderfully versatile — 肝 (kimo, liver / guts / core / essence). Idioms cluster around the courage meaning: 肝が据わる (kimo ga suwaru, "the liver settles in place" = to become composed and brave), 肝試し (kimodameshi, "test of liver" = a test of courage, especially the Japanese summer tradition of nighttime visits to haunted places, popular among schoolchildren and dating couples), 肝に銘じる (kimo ni meijiru, "to engrave on the liver" = to take deeply to heart). Japanese keeps liver-as-courage as a living metaphor more vividly than any other CJK language.

Memory aid: flesh (月) + shield (干) — the organ that shields the body and houses courage.