Skinna is an abbreviation of the derogatory "Skinner." It is used between skinnas and skinna allies as an affectionate greeting. It means someone who is uncircumcised.
A super-fly extreamly hot female that has a nice tight little body. It's more than an implication that the girl is in fact "skinny" but thats she has attractive feautures across the board.
"Dude my bitch gave me a Skinnamon Roll last night, it was so hot!"
"That's gross, we're not friends anymore; don't ever talk to me again."
"Whatever man!"