Etymology
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nimbus (n.)

1610s, "bright cloud surrounding a divine or sacred personage," from Latin nimbus "cloud," which is perhaps related to nebula "cloud, mist" (from PIE root *nebh- "cloud"). In art, the meaning "halo around the head of a representation of a divine or sacred person" is by 1727. Figurative use is by 1860. Compare aureole.

The nimbus of God the Father is represented as of triangular form, with rays diverging from it on all sides, or in the form of two superposed triangles, or in the same form (inscribed with the cross) as that of Christ. The nimbus of Christ contains a cross more or less enriched; that of the Virgin Mary is a plain circle, or occasionally a circlet of small stars, and that of angels and saints is often a circle of small rays. When the nimbus is depicted of a square form, it is supposed to indicate that the person was alive at the time of delineation. [Century Dictionary]
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nimbostratus (n.)

"thick, low, dark cloud from which rain or snow falls," 1932 (earlier use from late 19c. refers to different cloud types), from Modern Latin nimbus "cloud" (see nimbus) + connecting element -o- + stratus (see stratus). Nimbus as "a rain cloud" is attested from 1803. 

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*nebh- 
Proto-Indo-European root meaning "cloud."

It forms all or part of: nebula; nebular; nebulosity; nebulous; Neptune; Nibelungenlied; Niflheim; nimbus.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit nabhas- "vapor, cloud, mists, fog, sky;" Greek nephele, nephos "cloud;" Latin nebula "mist, vapor, fog, smoke, exhalation;" German Nebel "fog;" Old English nifol "dark, gloomy;" Welsh niwl "cloud, fog;" Slavic nebo.
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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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Agnus Dei (n.)
Late Latin, literally "lamb of God." From c. 1400 in English as the name of the part of the Mass beginning with these words, or (later) a musical setting of it. Latin agnus "lamb" is from PIE *agwh-no- "lamb" (see yean). For deus "god," see Zeus. The phrase is used from 1620s in reference to an image of a lamb as emblematic of Christ; usually it is pictured with a nimbus and supporting the banner of the Cross.
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cloud (n.)

Old English clud "mass of rock, hill," related to clod.

The modern sense "rain-cloud, mass of evaporated water visible and suspended in the sky" is a metaphoric extension that begins to appear c. 1300 in southern texts, based on similarity of cumulus clouds and rock masses. The usual Old English word for "cloud" was weolcan (see welkin). In Middle English, skie also originally meant "cloud." The last entry for cloud in the original rock mass sense in Middle English Compendium is from c. 1475.

The four fundamental types of cloud classification (cirrus, cumulus, stratus, nimbus) were proposed by British amateur meteorologist Luke Howard (1772-1864) in 1802.

Meaning "cloud-like mass of smoke or dust" is from late 14c. Figuratively, as something that obscures, darkens, threatens, or casts a shadow, from c. 1300; hence under a cloud (c. 1500). In the clouds "removed from earthly things; obscure, fanciful, unreal" is from 1640s. Cloud-compeller translates (poetically) Greek nephelegereta, a Homeric epithet of Zeus.

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