"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" (see in-(2)) + rigare "to water, to moisten," of uncertain origin, perhaps cognate with rain. Related: Irrigated; irrigating. In Middle English it was an adjective, "watered, flooded" (mid-15c.).