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).
"light carriage with low wheels either open or covered with a folding top," 1660s, from French calèche, from German kalesche, from Bohemian koleska, diminutive of kolesa "wheel-carriage," from kolo "wheel," from PIE root *kwel- (1) "revolve, move round." Also the name of the folding hood or top fitted to it (1856).
The Caleche is of French origin; a carriage with leather top, and portable glass shutters on the sides, and a panelled front, with sliding window. The whole front may be removed in a few minutes, making it an elegant open Barouche, with a half-top over the back seat. [Henry William Herbert ("Frank Forester"), "Hints to Horse-Keepers," New York, 1859]
also calcarious, "of the nature of lime, containing lime, chalky," 1670s, from Latin calcarius "of lime, pertaining to lime," from calx (genitive calcis) "lime, limestone" (see chalk (n.)).
The spelling in -eous, which appeared about 1790, is erroneous, influenced by words in -eous from Latin -eus. The etymological sense of calcar-eous would be 'of the nature of a spur.' [OED]