Night underpants -- generally fuller coverage than bikini, thong or other daytime underwear Akin to "sick underpants".
It's bedtime so where are my nunderpants?
nun.der.pants. underpants for nuns. similar to granny panties, but more holy. slang: nundies, not to be confused with dundies.
"I really wish I had a pair of lacy, black nundies"
"Did you see Sister Maria's nunderpants? She is so going to hell."