Which words only exist in the plural form?
There are quite a few words that fall into this category, for example:
- Names of devices:
- bellows, binoculars, forceps, gallows, glasses, pliers, scissors, shears, tongs.
- Names of items of clothing:
- braces, briefs, jeans, knickers, pants, pyjamas, shorts, tights, trousers.
It's true that some of these clothing words do become singular when they are used as modifiers (e.g. pyjama leg or trouser pocket). You'll also find a few of them used in what is sometimes humorously described as the 'fashion singular' (e.g. we're celebrating the return of the high-waisted trouser or another trend this season is the body-hugging jean) but this isn't yet accepted as part of standard English.
There is also the more difficult category of words ending in -ics which may or may not take a plural verb. If this type of word is used strictly for the name of a subject, it's regarded as singular, e.g. Economics, Ethics, or Physics. Compare with heroics or hysterics, for example, which are treated as plural.
Hustings, by the way, was once used in the singular, but only in Old and Middle English.