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]