You're taking a piss
Phrase used when expressing shock or confusion, as if saying "you're lying!" or "you can't be serious" or "i don't believe you!" in a positive way.
A: "i just got engaged!"
B: "stop! you're taking a piss!"
or
A: "i just got robbed"
B:"you're taking a damn piss, when did this happen??"
B: "stop! you're taking a piss!"
or
A: "i just got robbed"
B:"you're taking a damn piss, when did this happen??"