You argue that it would be wrong to stab my neighbor and take all their stuff. I reply that you have an ugly face. I commit the "ad hominem" fallacy because I'm attacking you, not your argument. So one thing you could do is yell "OI, AD HOMINEM, NOT COOL."

[...] What you need to do is go one step more and say "the ugliness of my face has no bearing on moral judgments about whether it is okay to stab your neighbor."

But notice you could've just said that without yelling "ad hominem" first! In fact, that's how all fallacies work. If someone has actually committed a fallacy, you can just point out their mistake directly without being a pedant and finding a pat little name for all of their logical reasoning problems.

-- TychoCelchuuu on Reddit