Slothfulness is the state of being slothful: not wanting to move, work, or try. It is a noun, and a rather formal, old-fashioned one. Teachers, preachers, and books use it; friends almost never say “my slothfulness” when they mean they stayed in bed. It names the habit or the quality, not one lazy afternoon. In older moral writing it is treated as a fault of character, the same sloth as the old “deadly sin,” not only a mood. Though actually, English already has easier words — laziness, idleness — and those are what most people use. Use this one when you are reading older prose, or when you want the heavy, almost moral tone. It is still not slovenliness: that word is about mess and not taking care, not about sitting still.