A southern's way of calling you an idiot.
If you live in or around or even visit TN, you will hear this often.
-A pregnant woman forgetting her keys or where she parked her car in a parking lot...
someone would look at you 'bless your heart'
-You show up to a Doctors visit on the wrong day or time-
the receptionist says 'bless your heart'
-Try to use a coupon that expired the day before... not knowing it
expired.. the cashier would say 'bless your heart'