Origin: The Walking Dead fan base.
Meaning: Turning into a greedy, self-righteous, murderous psychopath. Turning on your best friend and trying to kill him, killing a little girl with little to no thought, putting an end to your own mama's life after she's given birth to your sibling.
That kid, Carl, man. He went Full Shane and dropped his mama; he's gunna need some serious mental help when he grows up.
That bitch be crazy. She threw knives at me, smashed my XBOX, and completely destroyed my place; after going Full Shane on me, she calls the next morning for a booty call.