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c.1200, "lamentation, mourning, weeping; complaining, the expressing of complaints; a complaint; lover's complaint; accusation, charge," probably from an unrecorded Old English *man "complaint," related to Old English mænan "complain, moan," also "tell, intend, signify" (see mean (v.1)); but OED discounts this connection. Meaning "long, low inarticulate murmur from some prolonged pain" is first recorded 1670s, "with onomatopoeic suggestion" [OED].
mid-13c., "mourn (someone); regret, bewail;" c.1300, "to lament, grieve; utter moans;" probably from Old English *manan, related to mænan "to lament" (see moan (n.)). From 1724 as "to make a low, mournful sound." Related: Moaned; moaning.