Etymology
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irrigate (v.)
"supply land with water," 1610s, from Latin irrigatus, past participle of irrigare "lead water to, refresh, irrigate, flood," from assimilated form of in- "into, in, on, upon" (from PIE root *en "in") + rigare "to water, to moisten," of uncertain origin. Perhaps [Watkins] from PIE *reg- (2) "moist" (see rain (n.)). De Vaan offers as possibilities the root of regere "to direct, lead," on the notion of leading water onto the fields, or to the root of rigere "be stiff," literally "stretch." The first better suits the sense, but has phonetic problems.

Related: Irrigated; irrigating. In Middle English it was an adjective, "watered, flooded" (mid-15c.). Other adjectival forms have been irriguous (1650s), irrigative (1842), irrigatorial (1867).
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irrigation (n.)
1610s, "a supplying of water to land," also in medical use, "supply of a liquid to some part of the body," from Latin irrigationem (nominative irrigatio) "a watering, irrigation," noun of action from past participle stem of irrigare "lead water to, irrigate, flood" (see irrigate).
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rain (n.)

Middle English rein, from Old English regn "rain, descent of water in drops through the atmosphere," from Proto-Germanic *regna- (source also of Old Saxon regan, Old Frisian rein, Middle Dutch reghen, Dutch regen, German regen, Old Norse regn, Gothic rign "rain"), with no certain cognates outside Germanic, unless it is from a presumed PIE *reg- "moist, wet," which may be the source of Latin rigare "to wet, moisten" (see irrigate).

Rain dance "dance performed by a tribal group in hope of summoning rain" is from 1867; rain date in listings for outdoor events, giving an alternative date should rain interrupt them on the intended day, is from 1948. To know enough to come in out of the rain (usually with a negative) "take ordinary measures for one's protection" is from 1590s. Rain-shower is Old English renscur. Rain-gauge "instrument for collecting and measuring the amount of rainfall at a given place" is by 1769.

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water (v.)
Origin and meaning of water
Old English wæterian "moisten, irrigate, supply water to; lead (cattle) to water;" from water (n.1). Meaning "to dilute" is attested from late 14c.; now usually as water down (1850). To make water "urinate" is recorded from early 15c. Related: Watered; watering.
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leak (v.)
"to let water in or out" [Johnson], late 14c., from Middle Dutch leken "to drip, to leak," or from Old Norse leka, both of them related to Old English leccan "to moisten, water, irrigate" (which did not survive into Middle English), all from Proto-Germanic *lek- "deficiency" (source also of Old High German lecchen "to become dry," German lechzen "to be parched with thirst"), from PIE root *leg- (2) "to dribble, trickle." The figurative meaning "come to be known in spite of efforts at concealment" dates from at least 1832; transitive sense first recorded 1859. Related: Leaked; leaking.
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leach (v.)

"wash or drain by percolation of water, treat by downward drainage," by 1660s in cookery, perhaps from a dialectal survival from Old English leccan "to moisten, water, wet, irrigate," which, under Norse influence, became leak (v.). The word was used 18c. in technological senses, such as leach-trough, a device used in salt-works in which corns of salt taken from brine were set to drain  dry, after which they were called leach-brine. Related: Leached; leaching. Hence leach (n.) "a preparation made by leaching or straining" (1630s), in later use especially "a separation of lye or alkali in solution."

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