Literally "true" and used as a substantive, "that which is true," "truth," (Isaiah 65:16) a word used in strong asseverations, fixing, as it were, the stamp of truth upon the assertion which it accompanied, and making it binding as an oath, and you can find more about that here on st-takla.org on other commentaries and dictionary entries. Comp. (Numbers 5:22) In the synagogues and private houses it was customary for the people or members of the family who were present to say "amen" to the prayers which were offered. (Matthew 6:13; 1 Corinthians 14:16) And not only public prayers, but those offered in private, and doxologies, were appropriately concluded with "amen." (Romans 9:5; 11:36; 15:33; 16:27; 2 Corinthians 13:14) etc.
Main reference: Smith's Bible Dictionary (1860s)
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