A peignoir is a fancy, somewhat old fashioned nightgown or lightweight, loose robe. In the old days, women wore peignoirs when they knew they'd be seen in their night clothes. Ooh la la.
What you might call a housecoat or a dressing gown can also be called a peignoir. The origin of this word is curiously related to a lady's long hair and the time at the end of the day when she would comb or brush it. The Middle French peignouoir means "garment worn over the shoulders while combing the hair," from peigner, "to comb the hair." The Latin root is pecten, "a comb."