I will fuck you up!
means the person saying it plans to seriously injure or kill the person it is said too
Tom: Hey, dude, you want to fight? Hit me!
Bill: Man, You should walk away! I have a black belt in karate. I will fuck you up!
Bill: Man, You should walk away! I have a black belt in karate. I will fuck you up!
i just fucked you up
I literally just engaged in sexual intercourse with you, but it was over so soon, you didn't enjoy it. Sorry.
Sorry, I just fucked you up. I'll do better next time. I promise.
i know you fucked up
I know you fucked up
I know you fucked up