Don't Sell Me a Dog
This Victorian phrase means 'don't lie to me'. In the 1800's, it was common for dog salespeople to try to pass mutts off as pedigrees. This is supposedly the origin of the phrase (which was popular until the 1870's).
“You think I'd believe that you're a wizard who's also the cousin of royalty and Bill Gates' twin brother? Don't sell me a dog.”