Feed him (ψωμιζε αυτον). Quotation from LXX text of Proverbs 25:21. Present active imperative of verb from ψωμος, a morsel, and so to feed crumbs to babies, then to feed in general. In N.T. only here and 1. Corinthians 13:3.

Thou shalt heap (σωρευσεις). Future active of old verb σωρευω from σωρος, a heap. In N.T. only here and 2. Timothy 3:6.

Coals of fire (ανθρακας πυρος). 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.