to give notice, advice, or intimation to (a person, group, etc.) of danger, impending evil, possible harm, or anything else unfavorable: They warned him of a plot against him. She was warned that her life was in danger.
to urge or advise to be careful; caution: to warn a careless driver.
to admonish or exhort, as to action or conduct: She warned her employees to be on time.
to notify, advise, or inform: to warn a person of an intended visit.
to give notice to (a person, group, etc.) to go, keep at a distance, etc. (often followed by away, off, etc.): A sign warns trespassers off the grounds. A marker warned boats away from the dock.