Jamieson, Fausset and Brown on Matthew 13:42

42. And shall cast them into a furnace of fire--rather, "the furnace of fire":

there shall be wailing and gnashing of teeth--What terrific strength of language--the "casting" or "flinging" expressive of indignation, abhorrence, contempt (compare Psa 9:17; Da 12:2): "the furnace of fire" denoting the fierceness of the torment: the "wailing" signifying the anguish this causes; while the "gnashing of teeth" is a graphic way of expressing the despair in which its remedilessness issues (see Mt 8:12)!