Well Bless Your Heart!
If you've ventured below the Mason Dixon, surely you've heard this beloved phrase.  It means different things depending on the circumstances of the person being blessed.  Sometimes it is truly meant as a way to convey sympathy, but for the most part, the intention is to allow the blesser to walk away gracefully from a situation where the other speaker is indeed a bonafide idiot  

It is common for a Southerner not to want to offend the other person.  However, as my mother has said a million and one times, just because somebody has the audacity to say something does not mean you have to validate their remark with a response.  Sadly, there are those among us that have not been taught this lesson, which most of us call tact.  Instead, they are abrasive and clumsily trample through conversations inserting their ethnocentric beliefs, thereby offending every Southerner in their presence.  In these instances, it is not uncommon for the Southerner to dismiss them with, "Bless your heart."  

What is being left off is of the utmost importance.  "Bless your heart, you're stupid."  The final piece of the sentence is offensive and so most Southerners will not finish the sentence.  Welcome to the South!