a red flag
一個警示信號或預警信號
口語·書面語·表示警告或危險信號
英文釋義
something that indicates a potential danger or warning sign
例句
His constant lateness was a red flag that he wasn't reliable.
典故來源
源自軍事或交通中紅色旗幟作為警告信號,后引申為警示危險或問題的標志。