barleycorn

From barley +‎ corn.

• (UK) IPA^(key): /ˈbɑː(ɹ)liˌkɔː(ɹ)n/

barleycorn (plural barleycorns)

1. 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.

2. (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.

3. (architecture, woodworking) A small groove between two mouldings.