Let's face it, nobody likes to be insulted, even moi, but some use this very thing as a form of argument in more or less subtle forms, a logical fallacy of irrelevance known by the Latin, because yours truly feels like being a pedant, the Argumentum ad Hominem, or the Argument to the Person.
This tactic of argumentation is the counterpoint to the Argument from Authority, a sort of polar opposite to it on the spectrum of genetic fallacies - arguments that focus on the source of a claim rather than valid logic or evidence - and like it attempts to call attention to real or imagined characteristics of the subject in order to sidestep the argument being made, in this case negative or unfavorable ones.
A special case of this is a subset called Poisoning the Well, also referred to as the Circumstantial ad Hominem, made even before the opponent makes his argument. This takes the form of associating the target with someone or something that is widely regarded as unpleasant or evil, such as implying or stating a connection with, for example, Nazis or a well-known serial killer. The name of this subset derives from medieval Europe, when rumors abounded during outbreaks of plague that Jews were causing Christians to die from the epidemic by poisoning the local well-water, since the real vectors of the plague were unknown at the time. This differs from the usual form in that it can be made against both a person and an idea or belief.
In that hideous little abortion of a movie, Expelled, there was much use of this fallacy in the association of evolution with the Holocaust specifically and the Nazis in general.
The most common and least subtle form of this argument in its general form, often used by the unimaginative is the use of plain and simple abusive form used to call attention to perceived (real or imaginary) personal shortcomings as a cheap way to dismiss an argument without ever addressing it.
...or..."Your argument is wrong because you're a known religious fundamentalist."
Often, also used is an alleged conflict of interest, or personal prejudice, the Bias ad Hominem, indicating that the one this is used on is untrustworthy as an impartial source, or as is often the case of an American politician accusing his opponent of being a socialist or a fascist even when these claims are not only irrelevant but false."I don't have to listen to you because you're one of those Godless atheists."
Another is when critics of the modern anti-vaccination movement are implicated as paid shills in the pocket of the 'Big Pharma' conspiracy, and whose statements therefore must be taken with deceptive intent in mind.
Another is when mainstream scientists are accused of being afraid for their funding, careers, political agendas, and reputations and so ignore or hide 'the truth' of the paranormal or global cooling. These last two, by the way, are also referred to as an argument from conspiracy, or an appeal to motive, another subset of this post's fallacy of discourse, but involving alleged circumstances of self-interest.
But the Ad Hominem argument is not always a fallacy, and in the proper use may be a valid and effective form of argument when it is used to promote the goal of critical discussion rather than obstruct it.
For example, one reasonable use is in pointing out a likely conflict of interest regarding the statements of another when the subject’s questionable background, credibility and circumstances are also true, relevant, and kept in their proper context, such as a disgruntled ex-mob employee testifying in a criminal case when he has been given leniency or other favors for his testimony.
And this fallacy is more complex than one might think...
Less commonly known, and just as poisonous to an argument, is the positive ad hominem, which uses the same sort of reasoning, substituting alleged personal attributes or circumstances for relevant evidence in an argument, but this time uses positive traits, such as sincerity, kindness, or piety, though any virtuous trait will do, and thus shades into an argument from authority/appeal to virtue.