In a state of uncomfortable suspense.
Tenterhooks aren't connected with tents, nor are they the hooks used by butchers, as the common misspelling 'tenderhooks' might suggest. A tenter is a wooden frame, often in the form of a line of fencing, used to hang woollen or linen cloth to prevent it from shrinking as it dries. The tenterhooks are, not surprisingly, the hooks on the tenter used to hold the cloth in place.
Tenters are no longer everyday objects but a hundred years ago, in wool weaving areas like the North of England, they were a common sight on the land around the many woollen mills, called 'tenter-fields'. It is easy to see how the figurative expression 'on tenterhooks', with its meaning of painful tension, derived from the 'tenting' or stretching of fabric. The expression was originally 'on the tenters'. The English West Country playwright John Ford was the first to record that expression in the play Broken Heart, 1633:
Passion, O, be contained. My very heart strings Are on the Tenters.
Towards the end of the century the more accurate 'on the tenterhooks' began to replace the earlier phrase. This first example that I have found of it in print is in the 1690 edition of a periodical that was published annually between 1688 and 1693, The General History of Europe:
The mischief is, they will not meet again these two years, so that all business must hang upon the tenterhooks till then.