red rag to a bull

Possibly refers to the red cape used by matadors in bullfighting.

red rag to a bull (plural red rags to a bull or red rags to bulls)

1. Something that will enrage another particular person.

   Synonym: red rag

   Simply mentioning the word "nationalism" to him is like a red rag to a bull.

   • 1883, Mark Twain [pseudonym; Samuel Langhorne Clemens], chapter 6, in Life on the Mississippi, Boston, Mass.: James R[ipley] Osgood and Company, →OCLC:

     This was a red rag to the bull. He raged and stormed so (he was crossing the river at the time) that I judge it made him blind, because he ran over the steering-oar of a trading-scow.

   • 1886 May, Thomas Hardy, chapter XXX, in The Mayor of Casterbridge: The Life and Death of a Man of Character. […], volumes (please specify |volume=I or II), London: Smith, Elder & Co., […], →OCLC:

     Any suspicion of impropriety was to Elizabeth-Jane like a red rag to a bull.

   • 1873–1884 (date written), Samuel Butler, edited by R[ichard] A[lexander] Streatfeild, The Way of All Flesh, London: Grant Richards, published 1903, →OCLC:

     He did not say that he had been visiting among his neighbours; this to Pryer would have been like a red rag to a bull.

   • 2023 March 28, Graeme McGarry, “Scott McTominay earns place in history as Scotland stun Spain”, in The Herald‎:

     The rolling around and screams of anguish from Porro though were like a red rag to a bull for the Scotland support, who howled and harangued his every touch before he was withdrawn at the interval, presumably for his own safety.

• This is often used in constructions equivalent to make someone see red.
• This is often used as a simile with like a or as a.