mid-15c., back-formation from lamentation or else from Old French lamenter "to moan, bewail" (14c.) and directly from Latin lamentari "to wail, moan, weep, lament," from lamentum "a wailing, moaning, weeping." Related: Lamented; lamenting.
1590s, "expression of sorrow or grief," from Middle French lament and directly from Latin lamentum "a wailing, moaning, weeping" (see lamentation). From 1690s as "a mourning song."