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]
(..) ''lf you think that l was put on this earth to be your slave, you've got another think coming.'' And mothers are always more interested in the condition of your underwear than your body if you're ever in an accident. And they tell you that. ''l hope for my sake, if you're ever in an accident, you have on clean underwear.'' Well, l thought that's what an accident was. Look, you're driving a truck, here comes another truck gonna hit you. Now, whether you hit the truck or not, (..)1983 Bill Cosby: Himself