1. A phrase used to communicate "go away" or "shut up" in a very unfriendly manner. Related to piss off, bugger off, and go fuck yourself. As with many insults, it could be used between friends in a mock-offensive way. Example: Bob just kept rambling on and on so I finally told him to 'fuck off,' hoping he would leave but it didn't work and, well, that's why I killed him.
2. To masturbate. Related to "jack off" but usually with a harsh connotation. Unlike "jack off," which is mostly meant for males, "fuck off" is unisex. Be politically correct: Don't jack off -- Fuck off! This meaning can be used along with the first one for a combined message of "go away and masturbate" (because, conceivably, if someone is off masturbating somewhere, he/she won't be bothering whoever was being bothered), all in two little words. Example: I'm sick of you trying to get in my pants. Can't you just fuck off when you get home?
3. To waste time or do nothing significant. Similar to fucking around but without a connotation of having fun. Example: Quit fucking off and get back to work. This example could be used for the second definition as well, which is probably illegal if the place of work happens to be in public.
4. A substitute for the word good. This apparently originated in the UK and has spread to a few other English-speaking countries (Australia, for instance). Has yet to catch on in the US (I've never heard an American use fuck off like this). Example: That death's-head really makes for a fuck off-looking desk setup. Maybe Bob's untimely demise wasn't such a bad thing after all.