It's an offensive term to use at a person of Hindi or Muslim descent. Also a pretty funny prank call.
Me: Hello, I want to make a reservation.
Voshmi: Okay, how many people?
Me: Eight, and my name is Mitur
Voshmi: What, Mitur?
Me: Yeah It's M-I-T-U-R and my last name is B-I-N-E-S-D-E-R-T-Y. Can you please say it so I can see you got it right.
Voshmi: My Turban Is Dirty?
Me: Your turban is dirty!?!?!?
Voshmi: Fuck you motherfucker.
Me: Hello, is your chicken fresh?
Voshmi: Yes, it is fresh. How many pieces do you want?
Me: Is your wing sauce spicy?
Voshmi: Yes, IT'S SPICY! What do you want?
Me: Is your turban dirty.
Voshmi: YOUR MOTHER IS DIRTY!