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1550s, from Middle French myriade and directly from Late Latin myrias (genitive myriadis) "ten thousand," from Greek myrias (genitive myriados) "a number of ten thousand, countless numbers," from myrios (plural myrioi) "innumerable, countless, infinite; boundless," as a definite number, "ten thousand" ("the greatest number in Greek expressed by one word," Liddell & Scott say), of unknown origin; perhaps from PIE *meue- "abundant" (cf. Hittite muri- "cluster of grapes," Latin muto "penis," Middle Irish moth "penis"). Specific use is usually in translations from Greek or Latin.
c.1800, from myriad (n.).