the.com/nightdress

means A loose, lightweight gown worn to bed, typically by women or girls, in place of separate top-and-bottom pajamas.

from A straightforward compound of 'night' + 'dress,' arising in the 19th century as a common-sense label for a garment worn to sleep in — 'dress' here used in its older, broader sense of 'clothing' or 'attire' rather than specifically a frock. It sits alongside cousins like 'nightgown' and 'nightshirt,' all coined the same plain way: name the time, name the garment, done.