A variation of the heat index for weather. The fatman index measures how well each person handles the heat. Some people don't mind the heat, others fear it like grim death. The worse you handle heat, the higher the fatman index. This is based on the real lazy fat man who hates the heat and sweats like a pig. Originates from the area of Genoa, Illinois.
"This heat is terrible! fatman index is through the roof!"
"Better not go outside. It's hot and humid, not to mention the fatman index."