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.
c. 1400, "sad, sorrowful," from Latin lamentabilis "full of sorrow, mournful; lamentable, deplorable," from lamentari "to wail, moan, weep" (see lamentation). Early 15c. as "distressing, grievous." Related: Lamentably.
late 14c., from Old French lamentacion "lamentation, plaintive cry," and directly from Latin lamentationem (nominative lamentatio) "a wailing, moaning, a weeping," noun of action from past-participle stem of lamentari "to wail, moan, weep," from lamentum "a wailing," from an extended form of PIE root *la- "to shout, cry," which probably is imitative. De Vaan compares Sanskrit rayati "barks," Armenian lam "to weep, bewail;" Lithuanian loti, Old Church Slavonic lajati "to bark, scold;" Gothic lailoun "they scolded."
It replaced Old English cwiþan. The biblical book of Lamentations (late 14c.) is short for Lamentations of Jeremiah, from Latin Lamentationes (translating Greek Threnoi), from lamentatio "a wailing, moaning, weeping."