An attack upon an opponent's mom, in order to discredit their argument or opinion. This type of argument is usually kept in reserve until all forms of the ad hominem argument have been exhausted.
While the conversation can often degenerate into an amusing banter, the form of response is insidious and has entered all forms of dialogue.
Person 1: "The evidence suggests that the primary cause of Stalinistic dictatorships is a combination of socio-economic unrest and politically alien influences."
ad mominem response:
Person 2: "The evidence suggests...your mom!"