What does "bluebeard" mean?

Bluebeard: The hero of a mediæval French nursery legend, who, leaving home, enjoined his young wife not to open a certain room in his castle. She entered it, and found the murdered bodies of his former wives. -- Also used adjectively of a subject which it is forbidden to investigate. The Bluebeard chamber of his mind, into which no eye but his own must look. Carlyle.