Poking around, I see that that message first appears in 6.10.
Inform 6.10 release notes
(e) Dictionary words which end //p are marked as “pluralising”.
enters the dictionary as the word ‘grapes’, marked as pluralising. This does not mean that it’s a plural word in the ordinary linguistic sense. It means that it can refer to more than one game object.
Inform 6.10 release notes addendum
There are four new attributes:
(Well, actually “female” has been around for ages.) The first three aren’t useful in English-language games except to give the genders of people. (Note that you can give the “neuter” attribute to animate but sexless object such as a computer, if you so wish. This affects the handling of words like “it”, “him” and “her”.)
“pluralname” should be given to an object whose short name is linguistically a plural. For instance, an object called
“pillars of ice”
ought to have “pluralname”: this makes the library messages more grammatical. You might see something like
> OPEN PILLARS
Those aren’t openable.
instead of “That isn’t openable.”.
These are fringe benefits from the stricter sense of grammar which the multilingual parser can’t do without.
Given these things arriving at the same time, it seems plausible that the message could be a vestige of some abandoned-during-development scheme to directly associate gender with dictionary words via, e.g.,
//f, instead of just associating dictionary words with objects and assigning genders to objects. But, of course, I’m speculating.