The Collaborative International Dictionary of English v.0.48:

Duchy \Duch"y\ (d[u^]ch"[y^]), n.; pl. Duchies. [F. duch['e],
   OF. duch['e]e, (assumed) LL. ducitas, fr. L. dux. See
   Duke.]
   The territory or dominions of a duke; a dukedom.
   [1913 Webster]