red flag

¹ American  

noun

1. the symbol or banner of a left-wing revolutionary party.

2. a danger signal.

3. something that provokes an angry or hostile reaction.

   The talk about raising taxes was a red flag to many voters.

4. Also called powder flag.  Nautical. a red burgee, designating in the International Code of Signals the letter “B,” flown by itself to show that a vessel is carrying, loading, or discharging explosives or highly inflammable material.

5. (initial capital letters) a war game the U.S. Air Force holds several times each year at Nellis Air Force Base, Nevada, to train personnel in air combat.

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red-flag ² American  

[red-flag] / ˈrɛdˈflæg /

verb (used with object)

red-flagged, red-flagging

1. to mark or draw attention to for a particular purpose.

   The department has red-flagged the most urgent repair work to be done.

2. to provoke the attention of; alert; arouse.

   The animal's refusal to eat red-flagged the keeper that something was wrong.

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adjective

1. of or relating to a red flag.

2. intended or serving to emphasize, warn, incite, or provoke.

red flag ¹ British  

noun

1. a symbol of socialism, communism, or revolution

2. a warning of danger or a signal to stop

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Red Flag ² British  

noun

1. a socialist song, written by James Connell (1852–1929), Irish political activist, in 1889

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Etymology

Origin of red flag¹

First recorded in 1770–80

Origin of red-flag²

First recorded in 1880–85

Example Sentences

That is always a red flag for me when I’m reading an OLC opinion.

From Slate • Apr. 9, 2026

"He never had a single red flag at all."

From BBC • Apr. 7, 2026

On Feb. 4, it broke above a five-month cup-base pivot of $149.89, but the breakout quickly fizzled—a red flag.

From Barron's • Apr. 6, 2026

That should be a big red flag, and it bears discussing.

From MarketWatch • Apr. 6, 2026

Not sharing something so personal to me with the guy I was dating should’ve been a red flag about how positive I felt about our future.

From "What If It's Us" by Becky Albertalli and Adam Silvera