What is "sweating like a Mormon"?

Where I live it is rarely as hot as it is now and to boot we have had a flood, so the heat combined with the water has us coping with humidity and a bloom of mosquitoes.A lady recently moved to where I live, from somewhere in the Southern US, and we were outside trying to cope with the heat when she exclaimed I'm sweating like a Mormon", She did not know I am Mormon, and everybody looked at me. I just shrugged like no clue here what she meant.Thought I would ask on Y/A, since maybe its a Yankee thing, and I'm just a lowly Canuck.Also should I be insulted, or tell her that I'm LDS? Help, please.