A willfully infuriating or aggravating provocation; something that incites great anger or annoyance. An allusion to matadors' practice of waving red flags at a bull during a bullfight, though the color of the flag actually makes no difference to the animal. John's gone to gloat about his success over his brother, which is a red flag to a bull if I've ever seen one. You do realize that bringing your new girlfriend to your ex-wife's house is like a red flag to a bull, right?