What is the meaning of "fuck you up" ?

If you say it's an American slang, you should also know its meaning. 'Fuck' is the so-called four-letter word. If some one says,"I'll ***** you up." it means he is very mush annoyed or angry with you. The expression is very rude and it's not used in the educated circle. So, I think you should know an English idiom instead. "I will back you up" should be a much used phrase, meaning "I will support you."