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A way to ask who the hell someone is, where they disappeared to, or why they think they're so special, usually with a healthy dose of swearing.
'Fa's I boy?' I screamed at the guy who stole my fries.
I asked 'Fa's I boy?' when my friend vanished mid-conversation.
When the teacher called on a kid I didn't know, I muttered 'Fa's I boy?' under my breath.