red flag

A red flag is either a literal warning of some danger, like the signal flag used by a sinking ship, or a figurative warning, like the red flag a candidate's angry outburst sends to the voters about his temperament.

Red flags have signaled dangerous situations for centuries, from seventeenth century military flags marking the start of a battle to a lifeguard station's red flag, warning beachgoers about dangerous water conditions. Even some spam filters refer to their junk email alerts as red flags. Figuratively, behavior is often a red flag: "He ate all my food and left the toilet seat up — it raised a red flag about choosing him as a roommate."

Definitions of red flag
  1. noun
    a flag that serves as a warning signal
    “we didn't swim at the beach because the red flag was up”
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    type of:
    alarm, alarum, alert, warning signal
    an automatic signal (usually a sound) warning of danger
    flag, signal flag
    a rectangular piece of fabric used as a signalling device
  2. noun
    something that irritates or demands immediate action
    “doing that is like waving a red flag in front of a bull”
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    type of:
    annoyance, annoying, irritation, vexation
    the act of troubling or annoying someone
  3. noun
    the emblem of socialist revolution
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    type of:
    allegory, emblem
    a visible symbol representing an abstract idea
Word Family

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