A grain or “corn” of barley. [1913 Webster]
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.

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OXFORD DICTIONARY

barleycorn, n.
1 the grain of barley.
2 a former unit of measure (about a third of an inch) based on the length of a grain of barley.

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ROGET THESAURUS

barleycorn

Littleness

N littleness, smallness, exiguity, inextension, parvitude, parvity, duodecimo, Elzevir edition, epitome, microcosm, rudiment, vanishing point, thinness, dwarf, pygmy, pigmy, Liliputian, chit, pigwidgeon, urchin, elf, atomy, dandiprat, doll, puppet, Tom Thumb, Hop-o'-my-thumb, manikin, mannikin, homunculus, dapperling, cock-sparrow, animalcule, monad, mite, insect, emmet, fly, midge, gnat, shrimp, minnow, worm, maggot, entozoon, bacteria, infusoria, microzoa, phytozoaria, microbe, grub, tit, tomtit, runt, mouse, small fry, millet seed, mustard seed, barleycorn, pebble, grain of sand, molehill, button, bubble, point, atom, fragment, powder, point of a pin, mathematical point, minutiae, micrometer, vernier, scale, microphotography, photomicrography, micrography, photomicrograph, microphotograph, microscopy, microscope (optical instruments), little, small, minute, diminutive, microscopic, microzoal, inconsiderable, exiguous, puny, tiny, wee, petty, minikin, miniature, pygmy, pigmy, elfin, undersized, dwarf, dwarfed, dwarfish, spare, stunted, limited, cramp, cramped, pollard, Liliputian, dapper, pocket, portative, portable, duodecimo, dumpy, squat, short, impalpable, intangible, evanescent, imperceptible, invisible, inappreciable, insignificant, inconsiderable, trivial, infinitesimal, homoeopathic, atomic, subatomic, corpuscular, molecular, rudimentary, rudimental, embryonic, vestigial, weazen, scant, scraggy, scrubby, thin, granular, shrunk, brevipennate, in a small compass, in a nutshell, on a small scale, minutely, microscopically.

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