The most calculated "casual" garment in fashion — the negligee is etymologically "neglected" clothing, from the Latin for disregarding appearances.
A woman's light, filmy dressing gown or robe, typically worn over a nightgown.
From French négligée, feminine past participle of négliger meaning to neglect, from Latin neglegere meaning to disregard, from nec meaning not and legere meaning to pick up, choose Key roots: neglegere (Latin: "to disregard").
A negligee is literally "neglected" clothing — dress that has been carelessly thrown on, not formally assembled. What began as informal morning wear for French aristocrats became, by the 20th century, one of the most deliberately calculated garments in the wardrobe. Nothing casual about a modern negligee.