Undies is the friendly, slightly babyish word for underwear — pants, knickers, the things you wear next to your skin. It is affectionate and light, the word a parent uses with a small child ("go and put clean undies on"), or friends use with each other to take the seriousness out of a slightly awkward topic. Adults do use it, but always casually and usually with a smile; you would not find it in a shop sign, a laundry label, or anything written down officially — those say "underwear" or "underclothes". It is always plural in shape, like trousers and glasses: "my undies are in the wash", never "an undie". The word is more British and Australian than American, and it leans a little towards women's and children's underwear, though it is not strictly limited to those. Though actually, this whole corner of English is nervous — English speakers have invented a great many soft, joking words for underwear precisely because the plain word feels a bit exposed, and undies is one of those.