To smoke too much marijuana over a long period of time, or too much in a short, quick period, and vomiting; To become completely, and utterly fucked. Everything goes spinny, you feel dizzy and tingly, and eventually yak, and pass out.
Johnny: Bro, danny just yaked, do you think he's gunna Green-Out
Danny: Yo, slow down, i need to lie down man..
Rick: No man, you gotta stay awake.
*Danny passes out*