You should pack a 'change of underwear'. [=additional underwear that you can wear at another time]
He answered the door 'in his underwear'. [=while wearing only underwear]
(..) Yeah, see? That's why I hate it. So come on, you wanna go? Well, what about the sleeping arrangements in the cabin? Well... Same bed, and underwear and a T-shirt. - What about me? - Well, you'd be naked, of course. - Excuse me. Jerry Seinfeld? - Yeah. My names Sanger, Mel Sanger. (..)1992 TvShow: Seinfeld Title: The Bubble Boy Season: 4 Episode 7