SophistiCat: I believe in the Golden Rule - do to others as you would have them do to you.

Janus: That's bullshit. You beat your kids, treat your employees like crap, and cheat your customers.

Is that a legitimate argument? — T Clark

I would say that the fact that someone who propounds a principle may not follow that principle says nothing about the principle and everything about them. So, if they claim to believe in a principle that they apparently make no effort to practice then their claim to believe in the principle may indeed be bullshit, but not necessarily so.

An ad hominem argument does not have to be an insult. Here's one of my favorite ad hominem arguments. "But you're a cashier." Fairly long ( minute 30 seconds), so you might want to skip it. — T Clark

This could be a genetic fallacy - where the argument is rejected purely based on the source from which the argument is made. Some fallacies seem to overlap - the point being attack the argument, not the person making the argument.

Sometimes (often), the insulting party expects that the insultee will infer the intended argument, based on the discussion thus far. People usually don't speak in concise syllogisms, but use other forms of discourse, often skipping some steps (under the assumption that the reader will be able to correctly infer them themselves). When a discussion begins to contain insults, this can be taken as a clue to infer what argument is actually being made prior to that, it tends to be possible to (re)construct it. — baker

Not sure what you mean. Examples?

If their arguments are vacuous then they would be invalid and or unsound no? — Janus

Or simply not to your liking, but possibly still valid and sound.
I've never seen the term "vacuous" in literature about logic. It sounds more like a Jane Austen word, a haughty derision.

I think the point about the ad hominem fallacy is that it consists in assuming that someone's arguments are invalid or unsound or vacuous without examining their actual arguments.

Sure.

Like in the Seinfeld example mentioned above:
But you're a cashier!

The unspoken part we have to infer is:
You're a cashier, which is a lowly job not deserving respect, therefore, you're in no position to reject a romantic relationship with me on account that you don't respect my stand-up comedy act.