This strange contraption is called a builder's or painter's cripple. It's a wooden structure designed to offer a platform for workers to stand on while repairing or decorating windows. This one was found in the cellar at Brodsworth Hall in South Yorkshire, an English Heritage property which is being conserved 'as found'. There's an illustration of one in use in a painter's handbook from 1830.
I can find nothing about the word's etymology so I'm assuming it got its name because of what happened if a builder fell off one.