Or simply shortened to 'your mate?'
Is a phrase coined by the youth of sydney's north shore directed to someone that chimes in to a conversation/facebook status that is unwanted and/or completely unexpected.
Used to exclude the person and make them look like they don't belong in the situation i.e. to ask another person sarcastically "is he your mate" so as to say "WTF is he doing and is he even your mate?"
Thus make them realise they have completely chimed in unwanted.
Jack: "So yeah I was fucking Maia last night-"
Random: "YEAH I did her the other day too"
Jake: -pointing at random- "Is he your mate?"