The Collaborative International Dictionary of English v.0.48:

Rhenish \Rhen"ish\, a. [L. Rhenus the Rhine. ]
   Of or pertaining to the river Rhine; as, Rhenish wine. -- n.
   Rhine wine.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Rhenish
    adj 1: of or relating to the Rhine River and the lands adjacent
           to it; "Rhenish wines tend to be sweet"
    n 1: any of several white wines from the Rhine River valley in
         Germany (`hock' is British usage) [syn: Rhine wine,
         Rhenish, hock]