Definition of "warn"

(transitive) To make (someone) aware of (something impending); especially:

1. (transitive) To make (someone) aware of impending danger, evil, etc.  examples

2. (transitive) To notify or inform (someone, about something).  examples

3. (transitive) To summon (someone) to or inform of a formal meeting or duty.  quotations  examples

   Quotations

   • Committee being warned these following were absent or short [...]

     1741–2 March 4, Books of Keelman's Hospital, Newcastle, quoted in Northumberland Words (1894)

   • The people had been invited to the funeral, or warnt, by a special messenger a few days before the funeral took place.

     1874, Walter Gregor, An Echo of the Olden Time from the North of Scotland, page 142

   • [...] the plaintiff fraudulently warned the meeting for November 15, giving only five days' notice [...]

     1889, Reports of Cases Argued and Determined in the Supreme Court of the [State of] Vermont, page 490

4. (transitive, intransitive, of a clock, possibly obsolete) To make a sound (e.g. clicking or whirring) indicating that it is about to strike or chime (an hour).  quotations

   Quotations

   • Hark! the clock is warning ten;

     1885, Walter Towers, Poems, Songs, and Ballads, page 189

   • No, not a word more, Andrew; the clock has warned for nine, and I am off.

     1885, Emma Marshall, In the East Country with Sir Thomas Browne, page 106

   • The clock warned, and the hands pointed to a few minutes before the hour. The preacher looked towards it. "And, as you sit here," he cried," the Old Year is dragging out its last moments and the New Year is coming up —"

     1902, Violet Jacob, The Sheep-Stealers, page 399

(transitive) To caution or admonish (someone) against unwise or unacceptable behaviour.  examples

(chiefly with "off", "away", and similar words) To advise or order to go or stay away.  examples

(intransitive) To give warning.  quotations  examples