Meaning of “warn” - Learner’s Dictionary

warn

verb [ T ] us uk /wɔːn/
Extra Examples
Before I could warn him, he had fallen.He hopes his son's death will serve to warn others about the dangers of owning a gun.The doctors have warned him but he just carries on drinking.Police have warned that there are a lot of fake £50 notes in circulation.She was warned that her behaviour was unacceptable.
OF DANGER

B1 to tell someone that something bad may happen in the future, so that they can prevent it:

[ + that ] I warned you that it would be cold but you still wouldn't wear a coat.
I've been warning him for months.
ADVISE

to advise someone not to do something that could cause danger or trouble:

[ + to do sth ] I warned you not to tell her.

(Definition of “warn” from the Cambridge Learner’s Dictionary © Cambridge University Press)