a red flag
一个警示信号或预警信号
口语·书面语·表示警告或危险信号
英文释义
something that indicates a potential danger or warning sign
例句
His constant lateness was a red flag that he wasn't reliable.
典故来源
源自军事或交通中红色旗帜作为警告信号，后引申为警示危险或问题的标志。