1650s, "a heap," from Latin cumulus "a heap, pile, mass, surplus," from PIE *ku-m-olo-, suffixed shortened form of root *keue- "to swell" (compare Sanskrit svayati "swells up, is strong," Greek kyein "to swell," Lithuanian šaunas "firm, solid, fit, capable"). Meteorological use for "rounded mass of clouds" first attested 1803.