warn about (someone or something)
1. To provide information in advance about the risks or dangers associated with someone or something. Analysts have been warning about a likely downturn in the economy for months now, so this dip in the market shouldn't come as a surprise. Police are warning about a number of escaped inmates who are at large in the area.
2. To caution someone about someone or something; to inform someone about the risks or dangers of someone or something. In this usage, a noun or pronoun is used between "warn" and "about." My mom always warns me about the dangers of texting while driving. You need to warn him about Allison before he falls prey to her usual tricks.
warn someone about someone or something
to advise someone about the dangers associated with someone or something. Didn't I warn you about the dangers of going there? I warned you about Alice.
1. To make someone aware of the actual or potential harm, danger, or evil associated with something: I warned the kids about riding their bikes in the street.
2. To make aware in advance of some actual or potential harm, danger, or evil: The report warned about a possible attack.