Tenterhook

Tenterhooks are hooks in a device called a tenter. Tenters were originally large wooden frames which were used as far back as the 14th century in the process of making woollen cloth. After a piece of cloth was woven, it still contained oil from the fleece and some dirt. A craftsperson called a fuller (also called a tucker or wa[u]lker) cleaned the woollen cloth in a fulling mill, and then had to dry it carefully or the woollen fabric would shrink. To prevent this shrinkage, the fuller would place the wet cloth on a tenter, and leave it to dry outdoors. The lengths of wet cloth were stretched on the tenter (from Latin tendere, meaning "to stretch") using tenterhooks (hooked nails driven through the wood) all around the perimeter of the frame to which the cloth's edges (selvedges) were fixed, so that as it dried the cloth would retain its shape and size. In some manufacturing areas, entire tenter-fields, larger open spaces full of tenters, were once common.

By the mid-18th century, the phrase "on tenterhooks" came to mean being in a state of tension, uneasiness, anxiety, or suspense, i.e. figuratively stretched like the cloth on the tenter.