Etymology
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cumulus (n.)

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.

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cirrocumulus (n.)

also cirro-cumulus, type of cloud having the characteristics of both cirrus and cumulus clouds, 1803, from combining form of cirrus + cumulus.

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cumulo- (n.)

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.

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encumber (v.)
early 14c., "burden, vex, inconvenience," from Old French encombrer "to block up, hinder, thwart," from Late Latin incombrare, from in- "in" (from PIE root *en "in") + combrus "barricade, obstacle," probably from Latin cumulus "heap" (see cumulus). Meaning "hinder, hamper" is attested in English from late 14c. Related: Encumbered; encumbering.
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*keue- 
*keuə-, Proto-Indo-European root meaning "to swell," also "vault, hole."

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."
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halcyon (adj.)
"calm, quiet, peaceful," 1540s, in halcyon dayes (translating Latin alcyonei dies, Greek alkyonides hemerai), 14 days of calm weather at the winter solstice, when a mythical bird (also identified with the kingfisher) was said to breed in a nest floating on calm seas. The name of this fabulous bird is attested in Middle English as alcioun (late 14c.).

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.
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thunderhead (n.)
"high-piled cumulus cloud," one likely to develop into a thunderstorm, 1861, from thunder (n.) + head (n.).
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cumulous (adj.)

in reference to clouds, "of the nature of cumulus clouds," 1815, see cumulo- + -ous.

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cumulative (adj.)

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).

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