You cannot use numbers or the article "a" with plural nouns; however, you can use the expressions "a pair of panties", "two pairs of panties", etc.
It's "a pair of panties". In American English, when talking about the close-fitting underpants worn by girls or women, we use the plural noun "panties".
She has to buy a pair of panties.
She has to buy a panties.
You cannot use numbers or the article "a" with these plural nouns; however, you can use the expressions "a pair of panties", "two pairs of panties", etc.
She bought a pair of panties this morning.
There are three pairs of panties on the bed.
We commonly use a possessive pronoun, such as my, your, her, etc.
Jennifer keeps her panties in the drawer.
But omit the determiner to speak about panties in a general sense.
Panties are generally made of breathable material.
Remember, plural nouns, like "panties", take a plural verb, so we say "panties are", not "panties is".
Are your panties white?
Is your panties white?
Other articles of clothing that do not take the article "a" are listed below: