Originally Posted by
cXm0d
I think it might be because bra is a shortened form of brassiere, a French word. Panties is a cutesy variation of underpants - which describes how they are worn, as in under pants. Pant is sometimes a singular, and then there are trousers, which I've never seen as simply trouser unless used in conjunction with another word, like trouser snake. But now I've gone rather far afield, I fear, and brought up many details in my digression that you probably were not seeking.
(Googlemagic for that one)