百无禁忌

Recorded in Qing scholar Fan Yin's 《越谚》 (Yuè Yàn — Yue Proverbs): 百无禁忌，诸邪回避 ('all taboos lifted, all evils retreat'). Originally a Chinese New Year auspicious formula written on door slips to ward off bad luck during sensitive days when one might accidentally say the wrong thing.

usage

Modern usage often invites others to speak frankly: '今天百无禁忌，随便聊' ('today no topic is off-limits, chat freely'). Can also be slightly negative — describing someone reckless who 'has no scruples'.