Red rag to a bull

This expression meaning to provoke or enrage someone dates from the early 19th century. In the 16th century, red rag was slang for the tongue and if a person flapped his or her red rag, it meant that they were over-talkative. Red rag in this expression, however, is the literal one. Flapping a piece of red cloth at a bull is very likely to provoke and enrage it, as evidenced in modern bullfighting.