early 15c., "damage, state of adversity;" 1550s, "a great misfortune or cause of misery," from Old French calamite (14c.), from Latin calamitatem (nominative calamitas) "damage, loss, failure; disaster, misfortune, adversity," a word of obscure origin.
Early etymologists associated it with calamus "straw" (see shawm) on the notion of damage to crops, but this seems folk-etymology. Perhaps it is from a lost root also preserved in incolumis "uninjured," from PIE *kle-mo-, from *kel- "to strike, cut" (see holt). Calamity Jane was the nickname (attested by 1876) of U.S. frontierswoman, scout, and folk-hero Martha Jane Cannary (c. 1852-1903).