Negligee n. : An easy, unceremonious attire; undress; also, a kind of easy robe or dressing gown worn by women.

Example:
How do I choose a Negligee for my girlfriend?

Taxonomy
negligee, negligees, free encyclopedia, 1970s, nightwear, bows, fabrics, 1940s, clothing, search, young woman, négligé, in the bedroom, first introduced, 18th-century, mimicked, heavy head, head-to-toe.

Defination from the web:
... The modern negligee thus perhaps owes more to woman's fine bedjackets or bed-capes, and up-market slips than to the nightgown. ...