The word “Amen” is commonly used by Jews, Christians and Muslims. Although Christians believe it means “So be it” or “Verily” or “Truly”, it has the Hebrew meaning of “Trustworthy” or “Reliable”.
The first use of the word Amen in the bible is in:
Num_5:22 and the woman shall say: 'Amen, Amen.'
Christ used the word Amen on numerous occasions:
Joh_1:51 And he saith to him: Amen, amen, I say to you, you shall see the heaven opened and the angels of God ascending and descending upon the Son of man.
The very last word of the Bible is indeed Amen:
Rev 22:21 The grace of Our Lord Yeshua The Messiah be with all of his Holy Ones. Amen.

The oldest roots of Amen come from Egypt, Amun-Ra:

Amun-Ra was the chief of the Egyptian gods. In the early days of the Egyptian civilization, he was worshipped as two separate gods. Amun was the god who created the universe. Ra was the god of the sun and light, who traveled across the sky every day in a burning boat.
The Talmud teaches homiletically that the word amen is an acronym for:
God, King, Trustworthy - אל מלך נאמן https://www.linkedin.com/pulse/amen-cardinal-thomas-h-cavin/?published=t