You know why you think that? Because you are an idiot!

We have to take a stab at what the person means. If they mean, "You are an idiot, therefore, what you think is wrong" then we are looking at an Ad Hominem (Abusive) . However, if they mean "What you think is demonstrably wrong, but you are an idiot so it makes sense that you would think it is right" then it is just being rude and abusive, but not fallaciously.

The beauty and charm of logical analysis is that we can detect the fallacies and clarify which are true fallacies and which are not, by doing the same thing when we agree with the sentiment as we do when we do not.

"Rainman" was able to count the toothpicks in a dropped box at a glance, but could not even guess the cost of a house.  He was an idiot, and if he'd said the house cost $1.98, the quote would have been apt.

Conventionally, it's a mock of course, an Ad Hominem (Abusive) attack that does not even address the issue.

I trust I can generally know the difference.

Ad hominem fallacy: the fallacy of attacking the character or circumstances of an individual who is advancing a statement or an argument instead of seeking to disprove the truth of the statement or the soundness of the argument.