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):

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

Bouvier's Law Dictionary, Revised 6th Ed (1856):

BARLEYCORN. A lineal measure, containing one-third of an inch. Dane's Ab. c.
211, a. 13, s. 9. The barleycorn was the first measure, with its division
and multiples, of all our measures of length, superfices, and capacity. Id.
c. 211, a. 1 2, s. 2.