this is what you say to someone who is obviously lying or giving you an answer you find unacceptable. It can also be used as an exclamation should you find yourself in an unfair situation.
kid one : could you cover my shift on thursday, I heard you didn't have any plans.
kid two: I can't. I have to meet someone.
(knowing they have no plans whatsoever)
kid one : I see you've been snackin on those bullshit sanwiches.
you've had your car taken away by your parents, even though youre 21. You exclaim "bullshit sandwiches!"