1650s, "a heap," from Latin cumulus "a heap, pile, mass, surplus," from PIE *ku-m-olo-, suffixed shortened form of root *keue- "to swell." Meteorological use for "rounded mass of clouds, snowy white at the top with a darker, horizontal base" is attested by 1803.
word-forming element meaning "like or of the nature of cumulus clouds," 1887, from cumulus. Cumulo-stratus is from 1803; Cumulo-nimbus is attested by 1859.
It forms all or part of: accumulate; accumulation; cave; cavern; cavity; coeliac; church; codeine; coelacanth; coeliac; coelomate; concave; cumulate; cumulative; cumulus; enceinte; excavate; kirk; kymatology; Kyrie eleison.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit svayati "swells up, is strong;" Greek kyein "to swell," koilos "hollow, hollowed out, spacious, deep;" Latin cumulus "a heap, pile, mass, surplus;" Lithuanian šaunas "firm, solid, fit, capable;" Middle Irish cua "hollow;" Armenian soyl "cavity."
The name is from Latin halcyon, alcyon, from Greek halkyon, variant (perhaps a misspelling) of alkyon "kingfisher," a word of unknown origin. The explanation that this is from hals "sea; salt" (see halo-) + kyon "conceiving," present participle of kyein "to conceive," literally "to swell" (see cumulus) probably is ancient folk-etymology to explain a loan-word from a non-Indo-European language. Identified in mythology with Halcyone, daughter of Aeolus, who when widowed threw herself into the sea and became a kingfisher.
c. 1600, "formed by adding to, increasing inorganically" (a sense now obsolete), from Latin cumulatus, past participle of cumulare "to heap," from cumulus "heap" (from suffixed form of PIE root *keue- "to swell") + -ive. Meaning "increasing by successive additions" is from 1640s (implied in cumulatively).