"You are so dumb!" What I said is not an “ad hominem” if it is true. If it's true, then it is a “rebuke.” There are many dumb people in the world that don't know the difference between the two. An “ad hominem” is a fallacy (means it's false,) not a true statement. Rebukes are sharp negative criticisms, including comments about someone's character, that are true, not fallacies like “ad hominems.” When people block you or demonize you for using "ad hominems" when you are really rebuking them, then they are dirty little beasts who can't handle truth and logic. By the way, rebuking is the law in Abrahamic religions.
I can't stand people who are deliberately ignorant. I have found that some people do it so that they can have an excuse if something goes wrong. They ignore the truth and then say they didn't know they were suppose to know that as an excuse. The word came from biblical usage. In the Bible under the law, you cannot bear false witness (ad hominems), but you must rebuke people. The biblical definition requires us to separate fallacy from truth under the law. We are not allowed to rebuke people with fallacy, so the most frequent usage of the word and it's semantics is tightly connected to the requirement of truth.
