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]
(..) - But your dad's actually kind of cute. - - Shut up! - Come on. Like you've never sneaked a peek at him in his underwear? I bet he's got a big dick. I would. I would suck your dad's big fat dick, and then I'd fuck him until his eyes rolled back in his head. Jane, shh. What was that noise? (..)1999 American Beauty