warn S3 W2 /wɔːn $ wɔːrn/ BrE AmE verb [intransitive and transitive]
[ Language: Old English ; Origin: warnian ]
1 . to tell someone that something bad or dangerous may happen, so that they can avoid it or prevent it:
‘Be careful, the rocks are slippery,’ Alex warned.
warn somebody about something
Travellers to Africa are being warned about the danger of HIV infection.
warn (somebody) of something
Salmon farmers are warning of the severe crisis facing the industry.
warn somebody (not) to do something
I warned you not to walk home alone.
Motorists are being warned to avoid the centre of London this weekend.
warn somebody (that)
We warned them that there was a bull in the field.
2 . to tell someone about something before it happens so that they are not worried or surprised by it
warn somebody (that)
Warn her you’re going to be back late.
REGISTER
In everyday English, people often use the expression let someone know rather than warn someone:
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Let me know if you’re not going to finish on time.
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THESAURUS
▪ warn to tell someone about something bad or dangerous that might happen, so that they can avoid it or prevent it:
I warned you about sitting out in the sun too long.
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We were warned that there could be delays on the motorway, so we took another route.
▪ give somebody a warning to tell someone that if they continue to behave in an unsatisfactory way, they will be punished:
He’s already been given several warnings about handing in his essays late.
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The US gave a warning that if the hostages were not released, it would be forced to take military action.
▪ alert to officially or publicly warn people of possible danger so that they can prevent it or be ready to deal with it:
a campaign to alert people to the dangers of smoking
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An anonymous caller alerted the police that a bomb was due to go off.
▪ tip somebody off informal to secretly warn someone about something that is going to happen – used especially about warning the police about a crime:
The police found the drugs after being tipped off by local residents.
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Informants tipped the FBI off.
▪ caution formal to warn someone to do or not to do something in order to avoid a dangerous or bad result:
People are being cautioned against using credit cards abroad, in case of fraud.
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Health officials have cautioned the public to wash fruit thoroughly before eating it.
▪ forewarn /fɔːˈwɔːn $ fɔːrˈwɔːrn/ [usually passive] formal to warn someone about something that is going to happen, so that you are expecting it or ready for it:
We had been forewarned that the roads weren’t very good.
warn (somebody) against something phrasal verb
to advise someone not to do something because it may have dangerous or unpleasant results:
Her financial adviser warned her against such a risky investment.
warn (somebody) against doing something
The police have warned tourists against leaving the main tourist centres.
warn somebody ↔ away phrasal verb
to tell someone that they should not go near something, especially because it may be dangerous:
The snake’s markings are intended to warn away predators.
warn off phrasal verb
1 . warn somebody ↔ off to tell people that they should not go near something, especially because it might be dangerous:
Some animals mark their territory to warn off rivals.
2 . warn somebody off (doing) something especially British English to tell someone that they should not do or use something because it might be dangerous:
Doctors should have warned people off using the drug much earlier than they did.