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."