The Collaborative International Dictionary of English v.0.48:

Metheglin \Me*theg"lin\, n. [W. meddyglyn; medd mead + llyn
   liquor, juice. See Mead a drink.]
   A fermented beverage made of honey and water; mead. --Gay.
   [1913 Webster]

WordNet (r) 3.0 (2006):

metheglin
    n 1: spiced or medicated mead