Lose ten kilos in three days? Who would believe that.
Tips
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Colloquial and dismissive. Literal: '(only) a ghost would believe it' — i.e. no real person would. Pattern is highly productive: 鬼才知道 (who knows!), 鬼才管你 (who cares about you!), 鬼才去 (no way I'd go). Best for friends, not formal speech.
