late 14c., of the sea, "windless, without motion or agitation;" of a wind, "light, gentle," perhaps via Old French calme "tranquility, quiet," or directly from Old Italian calma "quiet, fair weather," which probably is from Late Latin cauma "heat of the mid-day sun" (in Italy, a time when everything rests and is still), from Greek kauma "heat" (especially of the sun), from kaiein "to burn" (see caustic). Spelling influenced by Latin calere "to be hot." Figurative application to social or mental conditions, "free from agitation or passion," is from 1560s.
c. 1400, "absence of storm or wind," from the adjective or from Old French calme, carme "stillness, quiet, tranquility," or directly from Old Italian (see calm (adj.)). Figurative sense "peaceful manner, mild bearing" is from early 15c.; that of "freedom from agitation or passion" is from 1540s.
Aftir the calm, the trouble sone Mot folowe. ["Romance of the Rose," c. 1400]
late 14c., "to become calm," from Old French calmer or from calm (adj.). Also transitive, "to make still or quiet" (1550s). Related: Calmed; calming.