Perhaps Pluralize Means Something Other Than I Thought

Besides the unfortunate use of a z where there really ought to be an s (yes, yes, I’m Australian), what does this word really mean?

pluralize v. tr.* To make plural.

• Grammar. To express in the plural.

OK, that’s pretty much what I thought it meant. So then riddle me this Batman:

simon$ ./script/consoleLoading development environment.>> "country".pluralize=> "countries">>

Good good. Just as expected. So, now let’s try something else:

>> "countries".pluralize=> **"country"**>>

Huh??? Since when did country become the plural of countries?