[1] : The Collaborative International Dictionary of English v.0.48

barleycorn \bar"leycorn`\, 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
see also:
corn john barleycorn 

[2] : WordNet (r) 2.0

barleycorn
     n 1: a grain of barley
     2: a grain of barley syn: barley
see also:
barley 

[3] : 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. 

Barleycorn >> B Words
Barleycorn, definition of term: Barleycorn
barleycorn_pag1.html

John Barleycorn >> J Words
John Barleycorn, definition of term: John Barleycorn
john+barleycorn_pag1.html