Plurals and singulars are not hewn in stone. Plurals, especially those of foreign extraction, are regularly mistaken for singulars, and – sometimes – vice versa.
The word stamina, for instance, was really the Latin plural of the word stamen (a term you may remember from biology class, albeit in a very different meaning), but has in English long been a singular. The same has happened with agenda and, more recently, data. It is happening under our eyes with phenomena. People get worked up about it, but there’s nothing new under the sun. Even the respectable opera was once a plural, and only became a singular because Italians couldn’t be bothered with Latin grammar – and why should they?
Another example, closer to home, is skates. It entered English from Dutch as a singular, but the final -s and the fact that these objects usually come in pairs conspired to get skates interpreted as an English plural in no time at all.
A very different piece of metal that often comes in pairs is the long bar that, fastened to sleepers, allows trains and subways to run from A to B: the rail. Having borrowed this word in the plural, Russians now call each of these bars a rels, with a regular plural relsy. (Or strictly speaking rel’s and rel’sy, where the apostrophe has nothing to do with possession or greengrocers.)
So far, I’ve only given examples of ‘number confusion’ with loanwords, but it happens with inherited vocabulary too.
The word truce started life as trewes, the plural of trewe, which is related to true. Since there is nothing particularly multiple about it, it began to be used as a singular, and truces has been a bona fide plural in its own right for centuries now.
The Latin words poma (for ‘apple’, earlier ‘fruit’) and folia (‘leaf’) also used to be plurals. But Latin words ending in -a could either be neuter plurals (as in paraphernalia, et cetera et cetera) or a feminine singular (as in vagina and formula). Poma, folia and a few other plurals were ‘reanalyzed’, as the jargon has it, and became feminine singulars, which in turn developed their own plurals.
Let’s focus on folia, because its further development was somewhat adventurous. First of all, it got mangled pretty badly in the process that transmogrified Latin into Spanish: the final result was hoja, plural hojas, which is pronounced as /ohas/. But what interests us here is that Chamorro, a language spoken in the Pacific island of Guam, borrowed this Spanish plural and once more turned it into a singular. Ohas is now a proper Chamorro word, not for ‘leaves’, but for ‘leaf’.
Unfortunately, I’ve not been able to establish beyond reasonable doubt what the Chamorro plural looks like; my best shot is ohas siha, but correct me if ‘m wrong. Anyway, I would suggest that we borrow the Chamorro word. We could drop the space in the middle and then use it for, say, ‘palm leaf’ – this is the Pacific, after all. And of course, we’d add an s for the plural: every palm tree has several ohassihas.
That’s the life for a linguist: drinking your cocktail on the beach under Pacific, Mediterranean or Caribbean ohassihas. It might not be something entirely new under the sun, but the rarity would be well worth the trip: the plural of an original plural, made singular three times over.