I've been reading a story (Beth by Bronte Follower). One thing that I find jarring about it is that he(?) continually uses "panty" when referring to women's underwear. For example, he writes "she wore a blue panty," or "she took off her panty."
I have never heard a woman, or man, use that in reference to women's underwear. It is always "panties" or "pair of panties, except in two instances. I have seen the singular used in an ad such as "one panty free when you buy two." There it saves space.
Otherwise, I have only seen it used as a descriptor, as in "panty-clad ass," "panty shield" or "pantyhose."
To me, it's the same as "pants," "underpants" or "knickers," when referring to the item, it's always plural.
Am I wrong?