Logical Fallacies only apply to arguments.
The phrase “You’re an idiot.” isn’t an argument, hence replying with “ad hom!” would be an incorrect use of language, because the definition of “ad hominem fallacy” is:
Ad hominem (
Latin for "to the person"
[1]), short for
argumentum ad hominem, is a fallacious argumentative strategy whereby genuine discussion of the topic at hand is avoided by instead attacking the character, motive, or other attribute of the person making the argument, or persons associated with the argument, rather than attacking the substance of the argument itself.
See, the definition specifically refers to it as a fallacy that applies to an argument. It uses the word “argument” more than once.
The sentence “Your argument for the existence of objective morality doesn’t work because you’re an idiot.” is an example of an ad hominem fallacy, because it’s fallaciously saying that an argument doesn’t work because of a reason (a low IQ) not related to the argument itself. It’s entirely possible that someone who has a low IQ could still be right about this particular argument.
So now that I’ve explained what a logical fallacy is as simply as I can, hopefully you’ll start using them correctly.