When cloth is fabricated, it is necessary or desirable to stretch the piece in order to even out the threads and to make the material straight.
Before the development of modern machine methods, whereby this is accomplished in the mill as a part of the routine of manufacture, it was the practice to stretch the fabric on frames, usually out of doors, which frames were known as tenters.
The frames were fitted with pins or hooks to hold the cloth, and these were, of course, tenterhooks.
The precise derivation of tenter is not well established, but it is fairly certain that it comes, eventually, from the Latin tendere, “to stretch,” through the French.
To be on tenterhooks is, literally, “to be stretched torturously, to be on the rack,” hence, figuratively, “to be subjected to agony approaching that of torture.”