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]
(..) ''A rolling stone gathers no moss''? Yeah. Does that mean something to you? lt's the same as ''Don't wash your dirty underwear in public.'' l'm not sure l understand what you mean. l'm smarter than him. ain't l? Well that. sort ot. has always meant... ...it's hard tor something to grow (..)1975 One Flew Over the Cuckoo's Nest