Literally 'heart lifted up, gallbladder dangling' - in Chinese medicine, the gallbladder (胆) is the seat of courage. When you're terrified, both your heart and courage are suspended in the air.
He walked through the dark alley with his heart in his mouth.
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In Chinese body metaphors, 胆 (gallbladder) represents courage and nerve. 胆大 means bold, 胆小 means timid, 有胆量 means having guts. This is why 吊胆 (dangling gallbladder) conveys fear - your courage is literally hanging by a thread.
