Etymology
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inundation (n.)

"an overflowing, a flood," early 15c., from Latin inundationem (nominative inundatio) "an overflowing," noun of action from past-participle stem of inundare "to overflow," from in- "onto" (from PIE root *en "in") + undare "to flow," from unda "a wave," from PIE *unda-, nasalized form of root *wed- (1) "water; wet."

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inundate (v.)
1620s, back-formation from inundation, or else from Latin inundatus, past participle of inundare "to overflow, run over" (source also of Spanish inundar, French inonder). Related: Inundated; inundating.
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*wed- (1)
Proto-Indo-European root meaning "water; wet."

It forms all or part of: abound; anhydrous; carbohydrate; clepsydra; dropsy; hydra; hydrangea; hydrant; hydrargyrum; hydrate; hydraulic; hydro-; hydrogen; hydrophobia; hydrous; Hydrus; inundate; inundation; kirsch-wasser; nutria; otter; redound; redundant; surround; undine; undulant; undulate; undulation; vodka; wash; water (n.1); wet; whiskey; winter.

It is the hypothetical source of/evidence for its existence is provided by: Hittite watar, Sanskrit udrah, Greek hydor, Old Church Slavonic and Russian voda, Lithuanian vanduo, Old Prussian wundan, Gaelic uisge "water;" Latin unda "wave;" Old English wæter, Old High German wazzar, Gothic wato "water."
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overflow (n.)

1580s, "act of overflowing, an inundation," also "the excess that flows over," from overflow (v.).

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diluvian (adj.)

"relating to or of the nature of a flood," 1650s, from Latin diluvium "flood, inundation," from diluere "wash away," from dis- "away" (see dis-) + -luere, combining form of lavere "to wash" (from PIE root *leue- "to wash"). Related: Diluvianism (1816) "geological theory supposing the occurrence of a former universal deluge."

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diluvial (adj.)
Origin and meaning of diluvial

"pertaining to a flood" (especially The Flood of Genesis), 1650s, from Late Latin diluvialis, from Latin diluvium "flood, inundation," from diluere "wash away," from dis- "away" (see dis-) + -luere, combining form of lavere "to wash" (from PIE root *leue- "to wash"). Related: Diluvian.

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cataclysm (n.)

"a deluge, a flood," originally especially "Noah's flood," 1630s, from French cataclysme (16c.), from Latin cataclysmos or directly from Greek kataklysmos "deluge, flood, inundation," from kataklyzein "to deluge," from kata "down" (see cata-) + klyzein "to wash," from PIE *kleue- "to wash, clean" (see cloaca).

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diluvium (n.)

"coarse, detrital material" apparently deposited by powerful operation of water on a vast scale, 1819, from Latin diluvium "flood, inundation," from diluere "wash away," from dis- "away" (see dis-) + -luere, combining form of lavere "to wash" (from PIE root *leue- "to wash"). Middle English had diluvie "a flood, a deluge" (early 14c.) from Old French diluvie and directly from the Latin word.

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deluge (n.)

late 14c., "an overflowing of water, a great flood, Noah's Flood in Genesis," from Old French deluge (12c.), earlier deluve, from Latin diluvium "flood, inundation," from diluere "wash away," from dis- "away" (see dis-) + -luere, combining form of lavere "to wash" (from PIE root *leue- "to wash"). Figurative sense of "anything that overflows or floods" is from early 15c.

After me the deluge (F. après moi le déluge), a saying ascribed to Louis XV, who expressed thus his indifference to the results of his policy of selfish and reckless extravagance, and perhaps his apprehension of coming disaster. [Century Dictionary]
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dog days (n.)

"period of dry, hot weather at the height of summer," 1530s, from Latin dies caniculares, the idea, though not the phrase, from Greek; so called because they occur around the time of the heliacal rising of Sirius, the Dog Star (kyōn seirios). Noted as the hottest and most unwholesome time of the year; often reckoned as July 3 to August 11, but variously calculated, depending on latitude and on whether the greater Dog-star (Sirius) or the lesser one (Procyon) is reckoned.

The heliacal rising of Sirius has shifted down the calendar with the precession of the equinoxes; in ancient Egypt c. 3000 B.C.E. it coincided with the summer solstice, which also was the new year and the beginning of the inundation of the Nile. The "dog" association apparently began here (the star's hieroglyph was a dog), but the reasons for it are now obscure.

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