forewarn
verb /fɔːˈwɔːn/
  /fɔːrˈwɔːrn/
[usually passive] (formal)Verb Forms
Idioms | present simple I / you / we / they forewarn |  /fɔːˈwɔːn/  /fɔːrˈwɔːrn/ | 
| he / she / it forewarns |  /fɔːˈwɔːnz/  /fɔːrˈwɔːrnz/ | 
| past simple forewarned |  /fɔːˈwɔːnd/  /fɔːrˈwɔːrnd/ | 
| past participle forewarned |  /fɔːˈwɔːnd/  /fɔːrˈwɔːrnd/ | 
| -ing form forewarning |  /fɔːˈwɔːnɪŋ/  /fɔːrˈwɔːrnɪŋ/ | 
- to warn somebody about something bad or unpleasant before it happens- be forewarned of something The commander had been forewarned of the attack.
- be forewarned that… They were forewarned that a storm was approaching.
 
Idioms 
forewarned is forearmed 
- (saying) if you know about problems, dangers, etc. before they happen, you can be better prepared for themTopics Dangerc2