A swindling con artist usually in the sales or medical profession performing a sham to gain money with trickery or deception. They are also commonly known as a "quack", like a doctor of questionable character.
In this episode when Carlton asks the mad doctor where he performed the operation before, he tells him, "In Bratislava. But in America, no one believes me. They call me charlatan". This is a direct reference to the public addressing the doctor as a "quack" and a con artist out for a quick buck.