noun. A sober person (walking,standing) surrounded by high people (sitting,lying-down).
This comes from the universal famous high joke, saying a colour (usually blue, mauve or green) during a lull in the conversation.
"Geen" is a common mispronunciation of green due to the effect some drugs can have on the speech centre, and similarly, eye sight, particularly the interpretation of colour, which is often affected at the same time.
When stoned, people frequently lie back or sit low, and often marvel at the ability and effort of people who walk around. In dim lighting situations, these people will appear tinged green, and so exclaim the colour, often mispronouncing. At some point, someone must have thought it was a noun, and then aptly named the sober people moving around them.
*Dude walks past some stoners in a dimly-lit room*
Stoner#1: Dude! You're standing up!
*general giggles, while the Dude is looking blank*
Stoner#2: Dude! Greeeeeen!
Stoner#2: Oh my god he's geen!
Stoner#3: He's geen?
Stoner#1: He's a geen!
*general giggles resume*