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