1. To make (someone) aware of impending danger etc.
    We waved a flag to warn the oncoming traffic.
  2. To caution (someone) against unwise or unacceptable behaviour.
    He was warned against crossing the railway tracks at night.
    Don't let me catch you running in the corridor again, I warn you.
  3. To notify (someone) of something untoward.
    I phoned to warn him of the rail strike.
  4. To give warning.

Verb (etymology 2)

  1. To refuse, deny (someone something).

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