1: phrase used by Southern women to excuse themselves for speaking ill of someone else.
2: an expression of sympathy or pity.
3: a polite way to respond to an ignoramus, particularly male, who showers upon you flattering but unwanted compliments.
1: "She's as ugly as a mud-fence, bless her heart."
2: "Well, bless your heart, that must have been terrible!"
3: drunk fella: "I think you're beautiful! Even if you were ugly, I'd STILL think you were beautiful!"
recipient: "...bless your heart."