forewarn
verb [ T ] uk/fɔːˈwɔːn/ us/fɔːrˈwɔːrn/
预先警告，事先告诫
to tell someone that something unpleasant is going to happen
[ + (that) ] The employees had been forewarned (that) the end-of-year financial results would be poor. 雇员们已得到预先警告，年底的财政状况会很糟。
Idiom
forewarned is forearmedidiom saying 先知先戒备;有备无患
said to mean that if you know about something before it happens, you can be prepared for it