Amen comes to English from Latin, however the root language of origin is said to be Hebrew or Aramaic. It is somewhat hard to directly translate the word into English so its simply borrowed.
The Hebrew word amen seems to be derived from a relative verb-‘aman, which means “he confirmed, supported, or upheld”. It is also related to the Hebrew word for truth (’emet), which bears the idea of certainty or dependability.
‘Amen is itself an interjection used to either agree with, approve, affirm or emphasize something that has been said. Thus Jesus begins some of his sentences by using “Amen, amen, i say unto you…”. Sometimes, because of the word’s relation to truth some translations render the “amen, amen” as “truly, truly” or “verily, verily”. Also “certainly, certainly” or “most assuredly” because it is also associated with the Hebrew terms for confirmation or dependability.
When after a prayer one responds “Amen” it serves as an affirmation of the agreement with the words and content of the prayer said. So it is sometimes translated as “So be it”, or as is mostly perceived as an expression of faith that God will hear and answer the prayer.
So the translation or meaning of the word depends on the context in which it is used.