“Amen” is a Hebrew word we say to affirm the truth of a statement or blessing made by another person, or to declare that that the Creator is faithful to keep promises made to us in the Torah.
The word "Amen" first appears in the Torah in the Book of Numbers, in chapter 5, verse 22: "And the woman shall say 'amen, amen'." The context there is that a woman is being administered an oath. Saying amen is her acceptance of the oath as true, and that she accepts the consequences of the oath if she is lying. In this sense, the word amen means "true."
The letters of amen, "alef mem nun," are also seen by our Sages as an acrostic hinting to the phrase "(K)el Melech Ne'eman" - "God, the faithful King."
Perhaps the first place it can be seen as said in a "religious" ritual is at the end of Psalm 41, which ends the first of the five books of Psalms. There, King David says, "Blessed is the Lord, God of Israel, forever and ever; amen and amen." This verse is very similar to what we call a "blessing," and it ends with "amen."
Sources indicate that amen was said after blessings at least as early as the beginning of the Second Temple period. There's no evidence that this was when it was first introduced, and it very likely goes back much farther.
