Underwear is the layer that outerwear is not allowed to be
Undergarments sit against skin—or in layers—to protect outer clothes from body oils and to shape comfort and modesty. They split roughly into torso covers and waist-and-leg covers, with hybrids doing both. English slang multiplies: undies, knickers, panties, lingerie, intimates, even Australian rhyming Reg Grundys.
An undershirt, called a vest in Britain, covers most of the torso, while underpants, drawers or undershorts cover the groin and usually the buttocks. Men today commonly wear boxer shorts or boxer briefs, women bras and panties, which the British call knickers, and both sexes wear T-shirts, sleeveless singlets and classic briefs. British English adds smalls, from the older smallclothes, and the historical unmentionables, while Americans sometimes call women's underwear delicates, after the gentle wash cycle they need.
The jobs are practical. Underwear keeps sweat and other bodily discharges off outer clothes, cuts friction against the skin, shapes the body and gives support. Bras support the breasts and briefs the male genitals; corsets act as foundation garments that also alter a woman's silhouette. Athletes add jockstraps with protective cups or sports bras that reduce strain on chest ligaments during jogging, and male dancers wear dance belts under tights.
Other uses are situational. In cold climates long underwear adds a warm layer, and divers pick undergarments for dry suits by matching insulation to expected water temperature and effort; astronauts wear them under space suits. Camisoles and slips give modesty beneath sheer clothes, some undergarments are meant mainly to entice, and certain kinds carry religious significance.
The line between under and outer keeps moving. T-shirts and some shorts work either way, suitable fabrics let underwear double as nightwear or swimwear, and the Museum at the Fashion Institute of Technology links the underwear-as-outerwear trend especially with the 1980s, while noting lingerie's long influence on fashion.
Source: Underwear
