If you’re thinking, wait, I thought it was tenderhooks, you’re not alone. Amy explains this early 18th century idiom that describes having a sense of anxious anticipation.

Image courtesy Wikimedia Commons, Creative Commons

https://en.wikipedia.org/wiki/Tenterhook

Tenterhooks or tenter hooks are hooked nails in a device called a tenter. 

Tenters were wooden frames which were used as far back as the 14th century in the process of making woolen cloth.

https://www.merriam-webster.com/words-at-play/on-tenterhooks-origin-meaning-phrase

A tenterhook is defined as “a sharp hooked nail used especially for fastening cloth on a tenter.