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1.
P12
slang. Person behaving in a childish and immature manner.

Word originates from Swedish Pojke (=boy) 12, as seen in online communities. Usually used in the e-gaming community to describe a person who annoys and insults other participants during a game, thinking they are more skilled and better than everyone else. The term can be used to describe any person under 18 years old, not just persons that are 12 years old.
That guy is such a P12 with his high pitched voice and his flameing

John is such a P12 thinking he's best at everthing, always spamming everyone
by Slicke July 03, 2010
 
2.
p12
The majority of the swedish internet user is a 12 year old pojke (boy) hence the "p12" who spams, acts immature and are just retarded
greta says: arne must be a p12!
arne says: fUK OF!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! BICH
by Chenquie June 30, 2009
 
3.
Nickname for asian kids whos names are way too long to pronounce. P being the first letter, and 12 being the total number of letters in said name.
Ginger: Hi my name is Ginger, what's your's?

P-12: Patikamanant

Ginger: What the fuck? I'm gonna call you P-12.

P-12: Cool, a nickname! Do you want to be BFFs?

Ginger: Sure do!

P-12: Sweet!
by Russel Crow 184 July 08, 2009
 
4.
P12
Going absolutely buck-wild crazy. This usually involves demonic cursing, deafening screams, racial slurs and death threats.
Mike - "Did you hear what happened to Trent the other day?"

Matt - "Yeah! I heard Paolo went total P12 on his ass, I think he summoned a demon with all that demonic screaming"
by silentseizures August 26, 2010