Argumentum ad hominem is a type of logical fallacy in which one attempts to disprove an argument by attacking its source. It isn't just a posh way of saying "personal attack", and for that matter isn't necessarily a personal attack at all. TVTropes puts it best:
"You're stupid, therefore your argument is invalid" is an ad hominem; "your argument is invalid, therefore you're stupid" (or "Your argument is invalid and you're stupid") is not.
And an insult in isolation certainly isn't. It doesn't take a Latin scholar to figure out that it's not an argumentum ad anything if there's no actual argument being presented. And in any case, there isn't even such a thing as an ad hominum.