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."