red flag

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red flag (plural red flags)

1. (literally) A flag that is red, especially associated with leftist revolutionary ideologies.
   • 2018 November 11, “Polish government officials march with far-right groups to mark century of independence”, in The Daily Telegraph‎, archived from the original on 2023-03-07:

     Some chanted: "Pride, pride, national pride" and "Poland should be national not red or rainbow-coloured", in a reference to the red flag of the Soviet Union and the symbol of gay pride.

   1. (motor racing) Used as a signal that the race must be stopped immediately (and possibly restarted).
      • 2005, James A. Vlasich, Horsehide, Pigskin, Oval Tracks and Apple Pie, →ISBN, page 177:

        Typically, a red flag is used when it rains or when the safety of the clean-up crews is in danger because there is too much debris or oil on the track.

      • 2017, Jill Sherman, Racing Personal Watercraft (Sea-Doos), →ISBN, page 37:

        A red flag may be waved after a bad start or if a rider is injured. If you see a red flag, stop immediately and return to the starting line.

      • 2020 September 13, Andrew Benson, “Tuscan Grand Prix: Lewis Hamilton claims 90th win after incredible race”, in BBC News‎, archived from the original on 2023-04-03:

        The race was punctuated by two red flags, the first caused by a pile-up on the pit straight, the second by a high-speed crash for Lance Stroll.

2. (informal) A cue, warning, or alert; a sign or signal that something is wrong.

   Antonym: green flag

   Improper formatting is a red flag to look for other problems with the data.

   • 2016 July 9, Angelina Chapin, “Text therapy: once my therapist sent me an emoji, I knew it was game over”, in Katharine Viner, editor, The Guardian‎, London: Guardian News & Media, →ISSN, →OCLC, archived from the original on 2023-05-15:

     The first major red flag with Barbara was her use of emojis. After our second session, in which I listed my many negative qualities, she said my long-term mission was to accept myself – a statement she ended with a winking face.

   • 2021 June 19, David Vecsey, “Former Times Columnist George Vecsey Gets Tribute From Son”, in The New York Times‎, New York, N.Y.: The New York Times Company, →ISSN, →OCLC, archived from the original on 2023-01-07:

     In the pre-cellphone world of the early '80s, my dad might have done a few things that today would raise a few red flags, but in truth cultivated a sense of independence. "I'm headed to the ballpark," he'd say, dropping $20 on the desk in a Chicago hotel room. "Take the El to Addison, your ticket should be at Will Call. Try to find the media room after the game or just hang around outside the gate or just meet me back here."

   • 2021 October 5, Ekin Genç, “Investors Spent Millions on 'Evolved Apes' NFTs. Then They Got Scammed.”, in VICE‎, archived from the original on 2023-05-28:

     Evolved Ape investors noticed several red flags leading up to Evil Ape's rug pull. After the public sale on September 24, the announcements seemed suspiciously unprofessional and several of the leaders were not around anymore, one investor who requested anonymity due to the ongoing fallout from the scam told Motherboard. But they chalked it up to lack of experience at the time. "I don't think this giant storm was ever what was expected," the investor said.

3. (idiomatic, US) Something that will enrage a particular person; a red rag.
4. (rail transport, UK) A flag formerly used by a railway guard to warn the driver not to start the train.

• “red flag”, in Merriam-Webster Online Dictionary, Springfield, Mass.: Merriam-Webster, 1996–present.
• “red flag”, in Dictionary.com Unabridged, Dictionary.com, LLC, 1995–present.
• “red flag”, in Collins English Dictionary.
• “red flag, n.”, in OED Online ⁠, Oxford, Oxfordshire: Oxford University Press, launched 2000.
• “red-flag, v.”, in OED Online ⁠, Oxford, Oxfordshire: Oxford University Press, launched 2000.