LMAO. I actually haven't heard the term "underpants" since the 60s. They are generally: boxer shorts and briefs for men, panties for women, and the generic underwear. Plus a lot of other slang terms.
We certainly wear underpants in my neck of America, although also do use jockeys, briefs and underwear. Especially when speaking to children learning to wear same.
Hehe..I have my underpants on and there's nothing stopping me now. giggle
Yeah, this is a perfectly normal sentence to say to someone