To be on tenterhooks (not tenderhooks) is to be nervously waiting to find out what is going to happen in a tense or perilous situation.
Literally, a tenter is a wooden frame used to hang newly woven woolen cloth in order to prevent it from shrinking as it dries. The tenterhooks, obviously, are the hooks on the tenter used to hold the cloth in place. The figurative sense, which developed in the late 18th or early 19th century, comes from the fact that cloth hung on tenterhooks is tense and stretched. Those adjectives might also describe the mental state of someone in anxious suspense over something.
I hope (though the wish is a cruel one) that my fair readers, if any such readers have deigned to follow me thus far, are on tenterhooks to know to whom the prize was adjudged. [Monthly magazine or British register of literature, sciences, and the belles-lettres (1826)]
Now, Jim Fenton, you shall go along and tell your story, and not keep us on tenterhooks all night,” said Miss Buffum sharply. [Scribners monthly (1875)]
Not that it mattered, since there was literally nothing to be done before dusk; for it would be cruel to tell Kate and keep her on tenterhooks all day to no purpose.[On the face of the waters: a tale of the mutiny by Flora Annie Webster Steel (1897)]
Or was he … only puzzling Captain Anthony, the providential man, who, if he expected the girl to appear at any moment, must have been on tenterhooks all the time. [Chance, by Joseph Conrad (1913)]
Cuban exiles waited on tenterhooks Thursday for official details of an airlift which would supplant sea lanes as a refugee route out of Cuba. [AP via Montreal Gazette (1965)]
In Beijing, foreign journalists are on tenterhooks, wondering whether the expulsion this month of an American correspondent for Al Jazeera is a taste of things to come. [New York Times (2012)]