Middle voice from the same as hileos; to conciliate, i.e. (transitively) to atone for (sin), or (intransitively) be propitious -- be merciful, make reconciliation for.
see GREEK hileos
1. to render propitious to oneself, to appease, conciliate to oneself (from ἴλαος gracious, gentle); from Homer down; mostly with the accusative of a person, as Θεόν, Ἀθηνην, etc. (τόν Θεόν ἱλάσασθαι, Josephus, Antiquities 6, 6, 5); very rarely with the accusative of the thing, as τήν ὀργήν, Plutarch, Cat. min. 61 (with which cf. ἐξιλάσκεσθαι θυμόν, Proverbs 16:14 the Sept.). In Biblical Greek used passively, to become propitious, be placated or appeased; in 1 aorist imperative ἱλάσθητι, be propitious, be gracious, be merciful (in secular authors ἱληθι and Doric, ἵλαθι, which the gramm. regard as the present of an unused verb ἵλημι, to be propitious; cf. Alexander Buttmann (1873) Ausf. Sp. ii., p. 206; Kühner, § 343, i., p. 839; Passow, (or Liddell and Scott, or Veitch) under the word ἵλημι), with the dative of the thing or the person: Luke 18:13 (ταῖς ἁμαρτίαις, Psalm 78:9 2. by an Alexandrian usage, to expiate, make propitiation for (as ἐξιλάσκεσθαι in the O. T.): τάς ἁμαριτας, Hebrews 2:17 (ἡμῶν τάς ψυχάς, Philo, alleg. leg. 3, 61). (Cf. Kurtz, Commentary on Hebrews, at the passage cited; Winer's Grammar, 227 (213); Westcott, Epistles of St. John, p. 83f.)