To be in a deep coma or unconscious state, not waking up.
神志昏沉，不省人事。
清·石玉昆《三侠五义》第一回谁知早有人将信暗暗透于冷宫，秦凤一闻此言，胆裂魂飞，忙忙奏知李娘娘。李娘娘闻听，登时昏迷不醒。”
After the accident, he remained昏迷不醒 for three days.
Learn the next connected idiom: 昏迷不醒 (hūn mí bù xǐng) → 行之有效 (xíng zhī yǒu xiào)
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