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1.
A poser emo. The latest hip hop fashion trend in urban America, especially in the NYC area.

Posos generally wear:
•Expensive, bright, colorful, (ugly) sneakers (e.g. Nike Air Forces, Jordons); usually mids or highcuts. Usually doesn't match with the rest of the outfit. Also makes their feet look big.
•Tight jeans tucked under the tongue of the sneakers.
•Tight shirt or an ugly, graphic, polkadot printed hoodie. Perhaps a fancy belt for emphasis. Sometimes the shirt will match the colorful sneakers.
•Sometimes a colorful cap to top it off, usually matching the colorful shirt that matches the colorful sneakers.

Perhaps originating from Hip Hop artists wearing the same trends. They are called poser emos because the style is very similar to the emo style. This style is worn by both males and females of every race.
"How did that kid find a shirt and cap to match his rainbow sneakers?"
"Must be a poso!"

Google 'tight jeans colorful sneakers'
by Cheese Grater March 30, 2008
 
2.
POSOS means PIECE OF SHIT OPERATING SYSTEM.

Technically used to describe Vista, XP or MAC.
Man, this virus is messing up my POSOS.
by BoondockSaintsHooker January 18, 2010