c. 1200, mon, "lamentation, mourning, weeping; complaining, the expressing of complaints; a complaint; lover's complaint; accusation, charge," perhaps from an unrecorded Old English *mān "complaint," from mānan, a variant of mænan "complain, moan," also "tell, intend, signify" (see mean (v.1)); but OED discounts this connection. Meaning "long, low inarticulate murmur expressing grief or pain" is by 1670s, "with onomatopoeic suggestion" [OED].
mid-13c., monen, "mourn (someone); regret, bewail;" c. 1300, "to lament inarticulately, grieve; utter mournfully;" probably from Old English *mānan, a variant of mænan "to lament" (see moan (n.)). From late 14c. as "complain, tell one's troubles." From 1724 as "to make a low sound expressive of physical or mental suffering." Related: Moaned; moaning.