ἀμήν

Hebrew אָמֵן;
1. verbal adjective (from אָמַן to prop; Niph. to be firm), firm, metaphorically, faithful: ὁ ἀμήν, Rev. iii. 14 (where is added ὁ μάρτυς ὁ πιστός καί ἀληθινός).
2. it came to be used as an adverb by which something is asserted or confirmed:
a. at the beginning of a discourse, surely, of a truth, truly; so frequent in the discourses of Christ in Matthew, Mark, and Luke: ἀμήν λέγω ὑμῖν 'I solemnly declare unto you,' e. g. Mt 5:18; Mk 3:28; Lk 4:24. The repetition of the word (ἀμήν ἀμήν), employed by John alone in his Gospel (twenty-five times), has the force of a superlative, most assuredly: Jn 1:51 (Jn 1:52); .
b. at the close of a sentence; so it is, so be it, may it be fulfilled (γένοιτο, the Sept. Num 5:22; Deut 27:15, etc.): Rom 1:25; Rom 9:5; Gal 1:5; Eph 3:21; Phm 4:20; 1 Tim 1:17; Heb 13:21; 1 Pet 4:11; Rev 1:6, and often; cf. Jer 11:5; Jer 35:6; 1 Kgs 1:30. It was a custom, which passed over from the synagogues into the Christian assemblies, that when he who had read or discoursed had offered up a solemn prayer to God, the others in attendance responded Amen, and thus made the substance of what was uttered their own: 1 Cor 14:16 (τό ἀμήν, the well-known response Amen), cf. Num 5:22; Deut 27:15ff; Neh 5:13; Neh 8:6. 2 Cor 1:20 αἱ ἐπαγγελίαι... τό ναί, καί... τό ἀμήν, i. e. had shown themselves most sure. (Cf. B. D. under the word .)