Feed him (psōmize auton). Quotation from lxx text of Proverbs 25:21f. Present active imperative of verb from psōmos, a morsel, and so to feed crumbs to babies, then to feed in general. In N.T. only here and 1Co 13:3.
Thou shalt heap (sōreuseis). Future active of old verb sōreuō from sōros, a heap. In N.T. only here and 2Ti 3:6.
Coals of fire (anthrakas puros). That is, burning or live coals.
Anthrax (our “anthracite”) is an old word, only here in N.T. It is a metaphor for keen anguish. The Arabs have a proverb “coals in the heart,” “fire in the liver.” Such kindness may lead to repentance also.