Amen is a word that came to English from Latin, which got it from Greek, which got it from Aramaic, which got it from Hebrew (technically, Aramaic may have had it anyway, before it became the standard language of the Jewish people a few centuries before the time of Christ).
It is difficult to translate this word directly, which is often a reason that words are borrowed from other languages (i.e., if there’s no direct way to translate this foreign word, just borrow it).
The specific Hebrew word amen (’amen ) appears to be derived from a related verb--’aman , which means "he confirmed, supported, or upheld." This verb is also associated with the Hebrew word for truth (’emet ), which carries the idea of certainty or dependability (i.e., that which is true is that which is certain or dependable).
’Amen itself is an interjection used to agree with, affirm, approve, or emphasize something else that has been said. Thus when Jesus begins certain sayings by declaring "Amen, amen, I say to you . . . " various Bible translations often render the "amen, amen" different ways. Because of the word’s association with the Hebrew terms for truth, the double amen is sometimes rendered "truly, truly" or "verily, verily." Because of its association with the Hebrew terms for confirmation or dependability, one might also translate it "certainly, certainly" or "most assuredly."
When one says "amen" in response to a prayer, it serves as an affirmation of agreement with the content of the prayer (cf. 1 Cor 14:16)—in which case it is sometimes translated "So be it" (cf. CCC 2856)—or as an expression of faith that God will hear and act on the prayer.
Bottom line: Amen is an interjection associated with the Hebrew words for truth and dependability, it conveys the idea of agreement or emphasis, and its meaning can be translated different ways depending on the context.