Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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.

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

Source : WordNet®

duenna
     n : a woman chaperon