The Collaborative International Dictionary of English v.0.48:

Ducal \Du"cal\, a. [F. ducal. See Duke.]
   Of or pertaining to a duke.
   [1913 Webster]

         His ducal cap was to be exchanged for a kingly crown.
                                                  --Motley.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ducal
    adj 1: of or belonging to or suitable for a duke; "ducal palace"

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "ducal":
   aristocratic, chivalrous, exalted, genteel, gentle, gentlemanlike,
   gentlemanly, high, kinglike, kingly, knightly, ladylike, noble,
   of gentle blood, of rank, patrician, princelike, princely,
   queenlike, queenly, quite the lady, titled