Deshabillé / Négligè

from French deshabillé=undress
Originally morning or house wear, later informal (i.e. non-court) wear in general.

As fashion shifted, informal gowns became acceptable as street wear, from where they often progressed to court wear, often changing their originally loose shape to a tighter-fitting one in the process.

Most notable examples are the manteau, which went from morning dress to everyday wear between 1670 and 1710 (an on to court in England after that), and the contouche, acceptable in the street but not in court around 1720, but official court dress in the 1770s.

Madame Pompadour is often depicted in deshabillé in official portraits, illustrating the shift of meaning of the word from "house dress" to "not (quite) court dress".