Abbott-Smith Manual Greek Lexicon
ἀμήν (G281)
129 ἀμήν , indecl. (Heb. אָמֵן , verbal adj. fr. אמן , to prop , ni., be firm ), [in LXX: I Ch 16:36 , I Es 9:46 , Ne 5:13 8:6 , To 8:8 14:15 , III Mac 7:23 , IV Mac 18:24 (elsewhere א״ is rendered ἀληθινός , Is 65:16 ; ἀληθῶς , Je 35 (28):6 ; γένοιτο , Nu 5:22 , De 27:15 ff., III Ki 1:36 , Ps 40 (41):13 71 (72):19 105 (106):48 , Je 11:5 )*.] As adj. (cf. Is 65:16 ), ὁ ἀ. , Re 3:14 . As adv. , in solemn assent to the statements or prayers of another ( Nu 5:22 , Ne 5:13 8:6 etc.): ὁ ἀ. , I Co 14:16 ; similarly, at the end of one's own prayer or ascription of praise: Ro 1:25 15:33 , Ga 1:5 , I Ti 1:17 ; in the Gospels, exclusively, introducing solemn statements of our Lord, truly , verily : Mt 5:18 , 26 Mk 3:28 (v. Swete, in l.), Lk 4:24 , al.; ἀ. ἀ. , always in Jo 1:52 3:3 5:19 , al.; τὸ ναί , καὶ . . . τὸ ἀ. , II Co 1:20 (on usage in π. , v. MM, VGT , s.v.).
G. Abbott-Smith, A Manual Greek Lexicon of the New Testament (1922) — public domain