bullshit's ass
1.  An expression used to denote complete and utter disbelief or shock.
2.  A way to emphasize a staunch unwillingness to perform an act or express severe disagreeance.
Ex. 1:  Lori:  Today is my 35th birthday.
        Kris:  Bullshit's ass!
        Lori:  It's true.
        Kris:  You look like you're no older than your early twenties!
Ex. 2:  Girl 1:  So, you're going to call him back, even though he was a lying, cheating loser?
Girl 2:  Bullshit's ass I will!
