The beating down of standing corn by wind or storm. The word is derived from the Latin calamus (a stalk of corn). Hence, Cicero calls a storm Calamitosa tempestas (a corn-levelling tempest).
“Another ill accident is drought, and the spoiling of the corn; inasmuch as the word `calamity' was first derived from calamus (stalk), when the corn could not get out of the ear.” —Bacon.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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