Underclothes are the garments you wear underneath your regular clothes, next to your skin. They include items like underwear, undershirts, and socks. The word is slightly old-fashioned; today people more often say “underwear” or “undergarments,” but you'll still see underclothes in older books or formal writing.

These garments serve several purposes. They provide an extra layer of warmth in cold weather, protect your outer clothes from sweat and body oils (making your jeans or shirts last longer between washings), and add comfort by preventing rougher fabrics like denim or wool from rubbing directly against your skin.

The word can also refer to clothing worn as a base layer. When someone in a historical novel changes into their underclothes for bed, they mean the simple garments worn underneath their fancier day clothes. In Little House on the Prairie, Laura Ingalls Wilder describes the long underwear pioneer families wore in winter, which were definitely underclothes even though they covered arms and legs completely.