piss on my leg, and tell me it's raining
Disbelieving response to an attempt to mislead, equivalent to: "pull the wool over my eyes"
You tell me that you've worked as an airline pilot and a surgeon, too? Go on ... you'd piss on my leg, and tell me it's raining.
Don't piss on my leg and tell me it's raining.
Don't hurt me and then pretend like you had nothing to do with it.
You argued, privately, to each member of the committee that I should be fired and now you come by on my last day to tell me how sorry you are about what "happened." Don't piss on my leg and tell me it's raining.