The Collaborative International Dictionary of English v.0.48:

Duenna \Du*en"na\, n.; pl. Duennas. [Sp. due[~n]a, do[~n]a,
   fr. L. domina. See Dame.]
   1. The chief lady in waiting on the queen of Spain. --Brande.
      [1913 Webster]

   2. An elderly lady holding a station between a governess and
      companion, and appointed to have charge over the younger
      ladies in a Spanish or a Portuguese family. --Brande & C.
      [1913 Webster]

   3. Any old woman who is employed to guard a younger one; a
      governess. --Arbuthnot.
      [1913 Webster]

WordNet (r) 3.0 (2006):

duenna
    n 1: a woman chaperon

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "duenna":
   abigail, amah, au pair girl, ayah, betweenmaid, biddy, bodyguard,
   cavalier, chambermaid, chaperon, companion, conductor, convoy,
   cook, dame, educatress, escort, esquire, fellow traveler,
   femme de chambre, fille de chambre, gentlewoman, girl, governess,
   guard, handmaid, handmaiden, hired girl, housemaid, instructress,
   kitchenmaid, lady-help, lady-in-waiting, live-in maid,
   live-out maid, maid, maidservant, mistress, nursemaid, parlormaid,
   safe-conduct, schooldame, schoolmarm, schoolmistress,
   scullery maid, servant girl, servitress, shepherd, soubrette,
   squire, swain, tutoress, tweeny, upstairs maid, usher,
   waiting maid, wench