Merriam-Webster defines an ad hominem argument as one marked by an attack on an opponent's character rather than by an answer to the contentions made. [1]
As a point about logic, that is a description of a bad argument. As a tactic it is a description of where your attention goes: the attack is the bait, and defending yourself is the trap.
Why it is hard to leave alone
Recognising a personal attack is rarely the hard part. Leaving it unanswered is, because being publicly mischaracterised creates a pull toward correcting the record — and correcting the record means talking about yourself.
That is the mechanism this site is describing. Once you are explaining that you are not lazy, not paid, not stupid, not lying, the original subject has been dropped by mutual consent.
What it looks like
- The reply addresses your motives, competence or history rather than your point
- A specific accusation you feel obliged to deny, especially a near-miss one
- Something about you that is public and slightly embarrassing, raised as if relevant
- Praise-shaped versions: "I'd expect better from someone as smart as you"
- The escalation is personal while the topic stays untouched
The phrase covers more than the insult
Worth knowing before using the term as an accusation, because it is narrower in an argument than it looks and wider in the dictionary.
Merriam-Webster carries two senses. The one quoted above is the familiar one. The first sense is different: appealing to feelings or prejudices rather than intellect. [2] That describes a much broader move — any appeal aimed below the argument — and it does not require saying anything about the person at all.
The history is more surprising still. Merriam-Webster records that in centuries past the phrase referred to a method of persuasion in which one introduces issues relating personally to one's opponent — their habits, practices or circumstances — instead of sticking to principles or facts. [3] That is not a fallacy so much as a technique: arguing from what this particular person is committed to rather than from the matter in the abstract. The dictionary then describes the scope expanding until ad hominem came to mean an attack aimed at an opponent's character rather than their ideas. [4]
None of that makes the modern usage wrong — a term means what it now means. It is useful for a narrower reason. The version of the phrase people reach for in an argument is the later and more hostile of the dictionary's two senses, and "that's ad hominem" can do less work than the person saying it intends. The next section is where that matters.
The honest difficulty
Not every remark about a person is a fallacy, and treating it that way is its own error.
If someone is advocating a policy they will personally profit from, saying so is relevant. If a claim rests on someone's firsthand account, whether they were present matters. If a person has been wrong about the same thing repeatedly, that bears on how much weight to give them now.
The test is whether the point about the person is being offered as a reason the argument fails, or as information about how much to trust an unverifiable claim. The first is a fallacy. The second is ordinary judgement.
What to do
Do not defend yourself in the thread. This is the difficult part, because the urge is strong and the accusation may be plainly unfair. But a denial is a paragraph about you, which is the subject the attack proposed.
Restate the point once, without reference to yourself. "The claim was that X. That's still the claim." It is short, it does not engage the attack, and it puts the subject back in front of anyone reading.
Leave the attack to the room. Answering it puts the accusation into your own words as well as theirs, and gives the thread a second reason to keep discussing you rather than the subject.