having my ass

Having my ass is typically used in surprising situations that may or may not be unpleasant. It is very similar to the term "deadass" and these can often be used interchangeably

Cashier: The total for the three plain shirts comes out to $400
Person: Are you having my ass?! That's way too much I don't believe it

having my ass

Having my ass is a phrase used when you cannot believe something. It is typically used in somewhat unpleasant situations. It is similar to the term deadass and can be used interchangeably

Cashier: The total for the three shirts comes out to $400
Person: Are you having my ass?! That's way too much I don't believe it.