barleycorn

noun3

1. •
   A grain of barley.

   • The Town Mouse came, and they sat down to a dinner of barleycorns and roots, the latter of which had a distinctly earthy flavour. — 1912
2. •
   A small groove between two moldings.
3. •
   (no longer used) 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.

   • This trip to Italy has actually enlarged the diameter of my head thirteen barleycorns! — 1879