What is forewarn?

said to mean that if you know about something before it happens, you can be prepared for it

you know his secret now ! forewarned is forearmed

forewarn - meme gif

the state of being forwarned or warned in advance

I gave forewarnment that i was gonna bitch slap her before i did

To warn others prior to, and before the act of fornication.

Nathan, on realising he was going to bang the sexy chic he'd stolen off of his friend Michael, text his flatmates in an act of forewarnication.

inform (someone) of a danger or possible problem

I have come to forewarn you, there will be a huge storm coming