blue chamber

Etymology
From the tale of Bluebeard.

Noun

 * 1)  A forbidden room.
 * 2) * 1859 June 18, Charles Dickens, A Tale of Two Cities, Book the Second, Chapter VI, in All the Year Round, Volume I, Number 8, page 172:
 * On Sundays, Miss Pross dined at the Doctor’s table, but on other days persisted in taking her meals, at unknown periods, either in the lower regions, or in her own room on the second floor—a blue chamber, to which no one but her Ladybird ever gained admittance.
 * On Sundays, Miss Pross dined at the Doctor’s table, but on other days persisted in taking her meals, at unknown periods, either in the lower regions, or in her own room on the second floor—a blue chamber, to which no one but her Ladybird ever gained admittance.