The nice way to call someone an idiot

"Bless your heart" is not really a compliment. It sounds sweet as pie, and sometimes is said affectionately about pitiable situations, but it’s often acid-tongued because you’re pitiful and did something you shouldn’t’ve but were too dumb to know better, which is why it’s a perfect saying to export from a region that has hung on for dear life to its manners...

Remember when the Tangerine Turd dinged South Carolina's moron wingnut Nikki Haley by saying she was an embarrassment to the state?
And she said: Bless your heart.

“Bless her heart, she has no fashion sense at all” is a good example. There are other versions of this: “Dog love ’em” and “I’ll pray for you,” as is “How nice for you!”...
What it means is that the speaker thinks you’ve gotten yourself into a bad situation, but part or all of the reason is something your fault — either a choice you made, or the fact that you’re dumb. The sympathy is because you probably can’t help it. Or as we say in the South, you come by it honestly. As in, your parents are dumb, too...
“It’s an expression that has a couple of different implications that go with it, a couple different meanings,” Kirk Hazen, a linguistics professor at Western Virginia University and expert on American dialects, tells MEL by phone.
“And it all is related to power,” he says. “The power of who’s talking and who’s listening. I know it’s gotten really popular. In linguistics we call that ‘enregistered.’”
“On the one hand, it can be a sincere form of endearment and concern where you actually feel empathetic for the person,” he says. “But that isn’t the expression everyone likes to talk about. The one everybody likes to talk about is when the speaker in, say, some perhaps condescending or perhaps biting way, notes that the person is in a bad state. And it’s mostly used, at least historically, by women.”

There's more at the link above.