Jamieson, Fausset & Brown

if thine enemy hunger, &c.--This is taken from Pro 25:21-22, which without doubt supplied the basis of those lofty precepts on that subject which form the culminating point of the Sermon on the Mount. in so doing thou shalt heap coals of fire on his head--As the heaping of "coals of fire" is in the Old Testament the figurative expression of divine vengeance (Psa 140:10; Psa 11:6, &c.), the true sense of these words seems to be, "That will be the most effectual vengeance--a vengeance under which he will be fain to bend" (So ALFORD, HODGE, &c.). Rom 12:21 confirms this.