E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The female of don. The Spanish don is derived from the Latin dominus = a lord, a master. A duenna is the chief lady-in-waiting on the Queen of Spain; but in common parlance it means a lady who is half companion and half governess, in charge of the younger female members of a noblemans or gentlemans family in Portugal or Spain.
There is no duenna so rigidly prudent and inexorably decorous as a superannuated coquette.W. Irving: Sketch-Book (Spectre Bridegroom).