Abusive Ad Hominem is a special case of Ad Hominem ("Against the person") in which a claim or argument is rejected on the basis of some irrelevant fact about the author of or the person presenting the claim or argument. It has the following form:
The reason why it is a fallacy is that the character of a person do not (in most cases) have a bearing on the truth or falsity of the claim being made (or the quality of the argument being made). Sometimes, a personal attack is relevant: if someone is shown to be a pathological liar, one can legitimately evaluate his claim as doubtful. Since even pathological liars might make correct claims, such attacks can be considered as weak.
Bill: "I believe that abortion is morally wrong."
Dave: "Of course you would say that, you're a priest."
Bill: "What about the arguments I gave to support my position?"
Dave: "Those don't count. Like I said, you're a priest, so you have to say that abortion is wrong. Further, you are just a lackey to the Pope, so I can't believe what you say."
You may argue that God doesn't exist, but you are just following a fad.
"Albert Einstein claims that E=mc2, but we know he was a Jew."
Adds an Argument by Innuendo to an Abusive Ad Hominem.
Dick Jones claims he is a MD, but as he is a pathological liar with narcissistic delusions, the claim is probably false.
Here we have evidence, based on experience, that we should not believe what Dick Jones says at face value.
Avoid personal attacks and focus your attention on the content of the claim.
Identify the attack and show that the character of the person has nothing to do with the truth or falsity of the proposition being defended.