fa's i boy - A loud and rude way to question someone's identity, location

A loud and rude way to question someone's identity, location, or self-importance, often with a curse word thrown in for good measure.

'Fa's I boy?' I yelled at the guy who took my seat.

I said 'Fa's I boy?' when my cousin showed up out of nowhere.

When the dog ran off with my sandwich, I shouted 'Fa's I boy?'