Friend: I'm gonna go ask out my crush.
Me: *fake sneezes* Sorry, I'm allergic to bullshit.
The joke is that the person doesn't believe their friend will actually ask out their crush, so they pretend to sneeze and say they are allergic to 'bullshit' to sarcastically express their disbelief.
Ur mom
poop
no1
you just watched I-robot didn't you