1 slang The tongue.
2A piece of red cloth used to provoke an animal, especially a fowl or bull; (now chiefly figurative) a source of provocation or annoyance; something which excites violent indignation. Frequently in similative use, especially in "like a red rag to a bull".
With bulls the stimulus appears to be the movement of the rag, as they are blind to the colour red.
3 nautical slang, historical Also with capital initials. = "Red Ensign". Compare "red duster".
4 derogatory = "red flag".
Early 17th century. From red+ rag.