To be a willfully infuriating or aggravating provocation; to be something that incites great anger or annoyance. An allusion to matadors' practice of waving red rags at a bull during a bullfight, though in fact the color of the rag makes no difference to the animal. John's gone to gloat about his success over his brother, which is like a red rag to a bull if you ask me. You do realize that bringing your new girlfriend to your ex-wife's house is like a red rag to a bull, right?