- Ad hominem (Latin for "to the man") is a logical fallacy. The pfft of all knowledge defines ad hominem thusly:
- ...an attempt to link the validity of a premise to a characteristic or belief of the person advocating the premise. The ad hominem is a classic logical fallacy, but it is not always fallacious; in some instances, questions of personal conduct, character, motives, etc., are legitimate and relevant to the issue.
- One of the common errors made during discussions and arguments at Pharyngula is to confuse ad hominem with insult. An ad hominem response to an argument might be: "You're wrong because you're an idiot." However saying: "You're wrong because of this, that and t'other fact, you idiot" is not ad hominem but is insulting. 
The selective pruning probably sells this to the uncritical readership of FFTB as valid, even if the rest of the Wikipedia article says otherwise. It's probably of more value to point folks to RationalWiki whose definition is a bit harder to selectively deform -
- The phrase ad hominem argument (often called an ad hominem attack) comes from the Latin "at the person". It also sometimes applies to any argument that centres on emotive (specifically irrelevant emotions) rather than rational or logical appeal. It occurs when people who are unable to attack the argument itself resort to attacking the person making it. As such arguments have nothing to do with the topic, they have no weight or validity against the argument. This is the case even if the attack is true; two plus two still equals four even if the first person to point this out was the most morally reprehensible person to have ever lived. 
The custom FFTB definition is a nonsense and is nothing more than an exercise in self-justification for the general vacuity of its readership who continuously show absolute inability to defend any of their derangements using acceptable methods and thus have no tools other than direct personal abuse to resort to.