The floor was dirt. Only the wealthy had something other than dirt. Hence the saying, "Dirt poor." The wealthy had slate floors that would get slipperyin the winter when wet, so they spread thresh (straw) on floor to help keep their footing. As the winter wore on, they added more thresh until, when you openedthe door, it would all start slipping outside. A piece of wood was placed in the entrance-way. Hence: a thresh hold.