The Collaborative International Dictionary of English v.0.48:

Barleycorn \Bar"ley*corn`\, n. [See Corn.]
   1. A grain or "corn" of barley.
      [1913 Webster]

   2. Formerly, a measure of length, equal to the average length
      of a grain of barley; the third part of an inch.
      [1913 Webster]

   John Barleycorn, a humorous personification of barley as
      the source of malt liquor or whisky.
      [1913 Webster]

WordNet (r) 3.0 (2006):

John Barleycorn
    n 1: an alcoholic beverage that is distilled rather than
         fermented [syn: liquor, spirits, booze, hard drink,
         hard liquor, John Barleycorn, strong drink]