It was written in 1918 to be part of a musical called "Yip Yip Yaphank," but was cut. Composer Irving Berlin revised it in 1938 partly as a reaction to the rise of Hitler, and it became an instant hit.
I would say God bless your heart, when somebody did something really nice, something gentle, kind or sacrificial.
To say God bless your heart is not merely saying thank you, but also acknowledges...