The basic meaning of the Semitic root from which it is derived is “firm,” “fixed,” or “sure,” and the related Hebrew verb also means “to be reliable” and “to be trusted.” The Greek Old Testament usually translates amen as “so be it”; in the English Bible it has frequently been rendered as “verily,” or “truly.”
https://www.kingjamesbibleonline.org/Bible-Verses-About-Words-Of-My-Mouth/
Amen–to Praise God
Unfortunately, “Amen” has lost its sacred meaning and is overused–almost like a period to a sentence. But in ancient times, Amen was uttered carefully. For example, in the scriptures, we note that the people summoned the word “Amen” to confirm or to “make more sure” their devotion to and worship of God.