What's the meaning of "like a red rag to a bull"?

It means it is something that sets you off.

Toreros used to try and upset bulls into fighting by waving a red cloth in front of their eyes.
I think it was really the rag in general, but people for a long time thought that the colour red was what got them going.

So anything that sets you off can be like a red rag to a bull.

The common expression, by the way is "a red flag to a bull." I believe that it has been determined that bulls can't see the color red, and that it is the motion of the cloth waved at them which attracts their attention. Bohne is right about the popular perception, it's just that the popular perception is, apparently, wrong.

If I didn't know "red flag to a bull," I might not know "sets one off" either.

To "set off" in this context, ghqiuyun, means to make angry. Waving a flag at a bull makes him angry enough to charge at the torero or the toreador in a bullfighting ring.

In many Western cultures, red is associated with anger. Phrases like this one and "he saw red" are based on this association.

Thanks
I'm so grateful for all of your help! Thank you very much ! Wish you good luck!

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