I've never heard the expression but I guess it might mean that they'd be winding you up = trying to make you annoyed or lose your temper. It's one of those innovative metaphors that one hears, like shaking someone's tree, rattling someone's cage... I suppose you can invent your own image. A lever is used to move something or to create pressure, so this metaphor does make some sort of sense. It doesn't do much for me though.
Well, good luck with your rap. In this context it certainly must mean don't wind him up because he'll explode with rage if you do.
(I'm afraid I can't help you with loading a nine, unless he's got a 9mm. automatic pistol, something like that?)
andersxman, as I said elsewhere, this is not the best place to ask about modern slang. By the time we get to know of the meanings of slang words they've gone cold!
We're also not great on song lyrics or poetic use of regular words. Add slang to the mix and we really don't have much to say which could be of deep insight!
I have never heard this expression either, but it sounds very similar to one we have that is "push somebody's button." It's meaning is similar to what has been posted here. We often use it in the sense "He really knows how to push her button" meaning he knows how to manipulate her, just what to say or do to make her mad or upset.
I think this is borrowed from a slang of another culture As far as I know the lever means patience. The best way is to look at the lyrics and see the over all meaning then the intention of using that phrase can be extracted