Source : Webster's Revised Unabridged Dictionary (1913)

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

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

   {John Barleycorn}, a humorous personification of barley as
      the source of malt liquor or whisky.

Source : WordNet®

barleycorn
     n 1: a grain of barley
     2: a grain of barley [syn: {barley}]