Definition of "duenna"

'Madam, I have a secret to tell you.' Now the very word secret is enough to rouse any one's curiosity; and, giving a quick glance round to see if her duennas were on the alert, she prepared to listen, and I saw that her eye had caught sight of the letter.

1834, L[etitia] E[lizabeth] L[andon], chapter XXIII, in Francesca Carrara. […], volume III, London: Richard Bentley, […], (successor to Henry Colburn), page 193

Then he placed her in a house and shut her up in a chamber, appointing ten old women as duennas to guard her, and forbade her to go forth to the Seven Palaces.

1949, Joseph Campbell, The Hero with a Thousand Faces