One who has never done anything wrong (against their conscience) will not be startled by a knock at the door in the middle of the night. It describes someone who is open, upright, and at ease because they have a clear conscience, fearing no scrutiny or accusations.
他一向做事光明磊落，平生不作亏心事，夜半敲门不吃惊，所以面对调查显得非常坦然。
He has always been upright and open in his actions, never having done anything against his conscience, so a midnight knock wouldn't startle him; thus, he appeared very calm when facing the inquiry.
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