warn against (something)(redirected from warn him against)
warn against (something)
To caution (someone) not to do something that is dangerous or risky. A noun or pronoun can be used between "warn" and "against." My mother always warned me against hanging out with hoodlums like you, and now I know why. Doctors are warning against spending too much time outside during this week's heatwave.
warn someone against someone or something
to advise someone against someone, something, or doing something. We warned them all against going to the region at this time. I warned her against Gerald.
To advise someone that something is dangerous or problematic and should be avoided: I warned them against driving without seatbelts. The doctor warns against smoking.