When a person acts out of their character to impress others and appear bad ass or have bad ass like qualities, Hence the meaning of the fruit a watermelon referring to guys who act as if they are the hulk, a one man army, or unstoppable.
I.E (In explanation)
A watermelon being green on the outside (a foolish rookie, in-experienced,fake,attention seeking) and but not hardened or tough and soft&pink inside..like a vagina.
Fake tough guy AKA soon to be prison bitch.
Joey is a fuckin watermelon felon...dude tries to play so bad ass but isn't impressing anyone..i bet if he goes to prison he would get turned into a prag ASAP.
Dude chill the hell out, they can see how green you are you're not a badass i won't continue to hang out with you if you keep up the act when they expose you for how pink you are i don't want to be affiliated with the likes of you.