"These pants are not new."
"Pants" are underwear. So you're agreeing with ninjandrew that パンツ should be a correct translation for the sentence then? :P
Trousers! "Zubon" is borrowed from French (jupon/jupe). DuoLingo has "pantsu" for underwear but Japanese has another word for underwear and I believe they use "pantsu" for trousers as well. Someone feel free to correct me if I'm wrong! (-:
My teacher told me that pantsu can be underwear or pants but the emphasis in the word is different. The same way hashi is both bridge and chopsticks but pronounced slightly different. However I can never remember where the emphasis should be.