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I know it's some sort of chat, but what is it exactly?Phone Message?Slang for AOL IM?
It's means Private Message. PM people by clicking the persons name you want to talk to and go on message user and write a message.
It's another way of saying "pwned," derived from the term PMS, during which mothers and sisters -blam!- so much that your Halo skills are disrupted and therefore the enemy can "PM" you.
Thanks Halofan91!