Why: In the New Yorker story "The Godchildren" by Tessa Hadley:
Opening the windows, Vivien leaned her arms on the wrought-iron balustrade, looking out into the night like a heroine in a film. She was wearing a turquoise nightdress under a filmy lacy matching knee-length peignoir: the girls knew that she called it this. With the light behind her, she didn’t look bad.Answer: A loose negligee or dressing gown:
The More You Know: The word literally means "garment worn while combing the hair," from peigner "to comb the hair," from Latin pectin, "comb"