This is a device by which a speaker attempts to defeat an opponent not by rational arguments but by calling him names or smearing his reputation. Mudslinging of this sort is not worthy of anyone. Do not repeat a smear and do not believe it until the person under attack has had a full opportunity to reply or until you have examined the evidence.
Chapter 23 Language and Logic, in Warriner's English Grammar and Composition Revised Edition, Complete Course, John E. Warriner and Francis Griffith, 1965