Warn definition

warnian, from . Cognate with German .

To make (someone) aware of impending danger etc.

We waved a flag to warn the oncoming traffic.
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.
To notify (someone) of something untoward.

I phoned to warn him of the rail strike.
To give warning.
- 1991, Clive James, ‘Making Programmes the World Wants’, The Dreaming Swimmer, Jonathan Cape 1992:
  
  Every country has its resident experts who warn that imported television will destroy the national consciousness and replace it with Dallas, The Waltons, Star Trek and Twin Peaks.
- 1988, Salman Rushdie, The Satanic Verses, Picador 2000, p. 496:
  
  She warned that he was seriously thinking of withdrawing his offer to part the waters, ‘so that all you'll get at the Arabian Sea is a saltwater bath [...]’.
- 1973, Thomas Pynchon, Gravity's Rainbow, Penguin 1995, p. 177:
  
  She is his deepest innocence in spaces of bough and hay before wishes were given a different name to warn that they might not come true [...].
- 1526, Tyndale's Bible, Galatians II, 9-10:
  
  then Iames Cephas and Iohn [...] agreed with vs that we shuld preache amonge the Hethen and they amonge the Iewes: warnynge only that we shulde remember the poore.

The intransitive sense is considered colloquial by some, and is explicitly proscribed by, for example, the Daily Telegraph style guide (which prefers give warning).