The Collaborative International Dictionary of English v.0.48:

Malmsey \Malm"sey\, n. [OE. malvesie, F. malvoisie, It.
   malvasia, malavagia, fr. Malvasia, or Napoli di Malvasia, in
   the Morea.]
   A kind of sweet wine from Crete, the Canary Islands, etc.
   --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

malmsey
    n 1: sweet Madeira wine