Down here in the south, Fat wood is called "lighterd" as in :

Bubba, I'm gonna hit you up side the head with a lighterd knot if you don't leave my grits alone. <img src="/images/graemlins/grin.gif" alt="" />

Fat wood, lighter wood or "lighterd" is pine wood that has a large amount of accumulated resin. In the deep south, old long leaf pines will have very "fat" heart wood. When the tree dies or is cut down the sap wood rots away leaving the fat wood. Many types of pine tree will produce some fat wood as the result of injury. When the sap rots away you are left with the "lighterd knot".

There is so much tar in the wood that it preserves the wood. In my grandfather's days, long peices were split into fence posts. I have cleared old fence rows where the wire had rusted away and the "lighterd" posts were still sound.

Fat wood stumps and knots are very common in the deep south. Back when county folks all had wood stoves and fire places it was the main source of kindling for starting fires. It is easy to light and burns fast but it smells and produces a dense black smoke, so it was used to get your oak or hickory started.

Fat wood is also known as tar wood. If you heat it in a closed vessel the tar will run out. This was the source of tar used in sailing ships. One of the major reasons the British government was interested in establishing the southern US colonies in the 1700's was to establish a secure source of Naval Stores.