Learn English – What does “O.G.” stand for

Currently reading "Slang for the ages. It's swag. bae," by Kory Stamper from International New York Times (October 6, 2014), I came across this acronym O.G.

"It[swag]'s generally taken to be a shortened form of the verb
"swagger," which was used to denote a certain insolent cockiness by
William Shakespeare, O.G."

Definitions of O.G. I have found so far are the following two:

1. officer of the guard.
2. original gum: the gum on the back of a stamp when it is issued to the public.

I think it is the second one (only because the first one doesn't seem fit) but what does original gum really mean? I am still confused.

Maybe some explanation with the context will be greatly appreciated.