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Webster 1913 Edition

Noun.

[See

A grain or “corn” of barley.

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.

barleycorn ‎(plural barleycorns)

A grain of barley.
- 1912, V.S. Vernon Jones, Aesop's Fables:
  The Town Mouse came, and they sat down to a dinner of barleycorns and roots, the latter of which had a distinctly earthy flavour.
(obsolete) The length of such a grain; a unit of length of approximately one third (or sometimes one quarter) of an inch or eight millimetres, still used as a basis for shoe sizes
- 1879, Geo. Alfred Townsend, Bohemian Days:
  This trip to Italy has actually enlarged the diameter of my head thirteen barleycorns!
- 1907, Various, The Wit and Humor of America, Volume IV. (of X.):
  Then shewed the devil the booke unto the friar, and the friar saw it was an uncut unique of incalculable value; the height of it was half a cubit and the breadth of it the fourth part of a cubit and the thickness of it five barleycorns lacking the space of three horsehairs.
(architecture, woodworking) A small groove between two mouldings.