The history of this word is somewhat amazing. Most 16th century homes had dirt floors. Only the wealthy had something other than dirt, hence the saying "dirt poor." The wealthy had slate floors, which would get slippery in the winter when wet. So they spread thresh (the residue from when wheat or barley is threshed) on the floor to help keep their footing. As the winter wore on they kept adding more thresh until when you opened the door it would all start slipping outside. So a piece of wood was placed at the entryway, hence a "threshold."