An expression used to tell someone to keep their inappropriate, logically reasoned thoughts to themselves.
Mr. Smith: "Honey my poop is green today. I think the reason is because I ate a bowl of peas with last night's dinner."
Mrs. Smith: "Ew, reason that to yourself dear."
Ms. Princess: "I think the reason why Tommy couldn't get a boner last night is because I'm so incredibly good looking that it made him nervous."
Friend: "Psh, reason that to yourself (bitch)."