negligee- informal attire usually made of a sheer fabric.

negligee: a noun

E.g a robe, night gown

Helga Crane was described wearing it at an early part of the book while she is still at Naxos reading in her room.

“In vivid green and gold negligee and glistening brocaded mules, deep sunk in the big high-backed chair, against whose dark tapestry her sharply cut face, with skin like yellow satn, was distinctly outlined, she was- to use a hackneyed word- attractive.” (page 36)

the word comes from the French word négligé which literally meant neglected, which I found interesting. I am actually embarrassed, I knew this word, I just did not know that this was how it was spelled.