It's "underpants are". Being a plural noun, this word takes a plural form of a verb.
It's "underpants are". Being a plural noun, this word takes a plural form of a verb. In American English, "underpants" is a term that can be used to talk about both women's and men's underwear. In British English, however, this word refers to only men's underwear.
Your underpants are too loose.
Your underpants is too loose.
Plural nouns, like "underpants", cannot be used with numbers or the article "a"; nevertheless, you can introduce the expression "a pair of underpants". Keep in mind that "pair" is a collective noun that requires a singular form of a verb.
There is a pair of underpants under the bed. (not "There are...")
Follow the same rule with other verbs, such as have, fit, look, etc.
Your underpants fit perfectly. (not "fits perfectly.")
Remember, we always use a plural pronoun or determiner (they, these, those, etc.) with plural nouns.
Those underpants look great! Where did you buy them?
A partial list of clothing-related nouns that only have a plural form, and therefore take "are", not "is", can be found below: