Undies is an informal, playful word for underwear. When your mom reminds you to pack clean undies for summer camp, she means your underwear. The word sounds lighthearted and a bit childish, which is why adults often use it when talking to or about kids, or when they want to sound casual and friendly rather than formal.

You might hear someone say they need to do laundry because they're running out of clean undies, or a parent might ask if you remembered to change your undies this morning. The word works for any kind of underwear: boxers, briefs, or anything else worn under your outer clothes.

While undies and underwear mean exactly the same thing, undies feels more playful and comfortable to say. You probably wouldn't use undies in a formal situation, like writing a thank-you note to your grandmother for birthday clothes. But with friends or family in everyday conversation, undies is perfectly normal. Other casual terms include skivvies or just plain underwear, which sounds more grown-up and neutral.