To make (someone) aware of (something impending); especially:
To caution or admonish (someone) against unwise or unacceptable behaviour.
Don't let me catch you running in the corridor again, I warn you.
(chiefly with "off", "away", and similar words) To advise or order to go or stay away.
A sign warns trespassers off/away from the site.
To give warning.