It’s a tenterhook! – the devices by which cloth was attached to the drying frame or tenter. As the cloth dries the tenterhooks prevent the cloth from shrinking. Hence – on tenterhooks…and if you look at the map you may find tenterfields near wool producing mills.
So there you have it.
When I was a child, I helped my mother and grandmother stretch newly-washed lace curtains on a small variety of this,