red flag字面意思是红旗，引申为危险信号；惹人生气的事物。

【例句】
The abnormal bleeding is your body's own red flag of danger.
非正常的出血是你的身体发出的危险信号。

The mere mention of his hated cousin's name was like waving a red flag in front of him.
只要一提他那个讨厌的表兄弟的名字，他就生气。