DISBELIEF
take something with a pinch of salt and take something with a grain of salt
to listen to a story or an explanation with considerable doubt.You must take anything she says with a grain of salt. She doesn't always tell the truth.They took my explanation with a pinch of salt. I was sure they didn't believe me.
tell it to the marines
I do not believe you (maybe the marines will). (Informal.)That's silly. Tell it to the marines.I don't care how good you think your reason is. Tell it to the marines!
That'll be the day.
I don't believe that the day will ever come (when something will happen).Do you really think that John will pass geometry? That'll be the day.John graduate? That'll be the day!