From Rap Dictionary
To be crunk/krunk with a hostile intention. To be buck is to be hard to be ready to fight. "Knuck if you buck" means "hit me if you're hard". Knuck means to take a swing to hit. "Knuck if you buck" -- Crime Mob (Knuck If You Buck )
- To threaten by 'jumping' toward a person. e.g.: Roy Jones Jr.'s signature move was to buck at the opponent and after he flinched hit him. "We'll be knuckin if you buckin, you'll be duckin if you get too close" -- Crime Mob (Knuck If You Buck )
- To fight.
- To shoot. "He bulletproof everything 'case niggaz try and buck him." -- LLoyd Banks featuring 50 Cent (Warrior pt. II ) "Before I buck lead, and make a lot of blood shed" Big L ('98 Freestyle) 
- To cock your gun.