Kitto's Popular Cyclopedia of Biblial Literature
This word is strictly an adjective, signifying 'firm,'and, metaphorically, 'faithful.' Thus in Revelation 3:14, our Lord is called 'the amen, the faithful and true witness.' In Isaiah 65:16, the Hebrews has 'the God of amen,' which our version renders 'the God of truth,' i.e. of fidelity. In its adverbial sense Amen means certainly, truly, surely. It is used in the beginning of a sentence by way of emphasis—rarely in the Old Testament (Jeremiah 28:6), but often by our Savior in the New, where it is commonly translated 'Verily.' In John's gospel alone it is often used by him in this way double, i.e.'verily, verily.' In the end of a sentence it often occurs singly or repeated, especially at the end of hymns or prayers, as 'amen and amen' (Psalms 41:13; Psalms 72:19; Psalms 89:52). The proper signification of it in this position is to confirm the words which have preceded, and invoke the fulfillment of them: 'so be it.' Hence in oaths, after the priest has repeated the words of the covenant or imprecation, all those who pronounced the amen bound themselves by the oath (Numbers 5:22; Deuteronomy 27:15; Deuteronomy 27:26; Nehemiah 5:13; Nehemiah 8:6; 1 Chronicles 16:36; comp. Psalms 106:48).
Kitto, John, ed. Entry for 'Amen'. "Kitto's Popular Cyclopedia of Biblial Literature". https://www.studylight.org/encyclopedias/eng/kbe/a/amen.html.