Several parts combine into one character.

胸 is a phonetic-semantic compound: 月 (the "flesh" radical for body parts) + 匈 (xiōng, originally itself a pictograph of the chest). The character has a fascinating bifurcation history: the original 匈 once meant "chest," but historical drift redirected 匈 to a new meaning — "the Xiongnu," the nomadic peoples of the Eurasian steppe who threatened Han dynasty China (recorded as 匈奴, Xiōngnú). Once 匈 was occupied by an ethnonym, the body-part meaning needed disambiguation, so the flesh radical 月 was added to create 胸 specifically for "chest." This is a rare and instructive case of one character splitting into two distinct characters when an etymological ambiguity demanded resolution.

The character carries a beautiful semantic extension across all three CJK languages: from the literal chest cavity (the body) → the seat of feeling (the heart and mind). Almost universally, "what is in the chest" became "what one feels and thinks."

Korean reading "hyung." 胸部 (hyungbu, chest — medical), 胸圍 (hyungwi, chest measurement / bust size), 胸像 (hyungsang, bust statue / a sculpture from the chest up), 胸襟 (hyunggeum, "chest robe" = inner feelings / the depth of one's heart). The Korean phrase (hyunggeum-eul teonota, "to open the depth of one's chest") is a deeply Korean way to express "to speak openly from the heart."

Mandarin xiōng, 1st tone. 胸 (xiōng, chest), 胸口 (xiōngkǒu, "chest mouth" = the pit of the stomach / solar plexus), 胸怀 (xiōnghuái, "chest-bosom" = breadth of mind / aspirations / one's inner state — used in expressions of leadership and vision: 胸怀大志 xiōnghuái dàzhì, "to bear great aspirations in the chest"), 胸罩 (xiōngzhào, "chest-cover" = brassiere — modern medical/colloquial term). The metaphorical extension to mental capacity is perhaps strongest in Mandarin.

Japanese on-reading キョウ (kyō) — 胸部 (kyōbu, chest — medical), 胸囲 (kyōi, chest circumference). Kun-reading むね (mune) — 胸 (mune, chest / heart), 胸が痛い (mune ga itai, "my chest hurts" — used both literally for physical pain and emotionally for heartbreak). The compound 胸騒ぎ (munasawagi, "chest-disturbance" = an uneasy feeling, foreboding, premonition) is one of the most evocative Japanese emotional vocabulary items — capturing the exact bodily sensation of dread or anticipation. Note the alternative kun-reading むな (muna) used only as a prefix in compounds: 胸毛 (munage, chest hair), 胸元 (munamoto, the area at the base of the throat).

Memory aid: 月 (flesh) plus 匈 (originally chest, now means Xiongnu people) — body-part disambiguation forced the addition of the flesh radical.