at the beginning of a discourse - surely, truly, of a truth
at the end - so it is, so be it, may it be fulfilled. It was a custom, which passed over from the synagogues to the Christian assemblies, that when he who had read or discoursed, had offered up solemn prayer to God, the others responded Amen, and thus made the substance of what was uttered their own.
The word "amen" is a most remarkable word. It was transliterated directly from the Hebrew into the Greek of the New Testament, then into Latin and into English and many other languages, so that it is practically a universal word. It has been called the best known word in human speech. The word is directly related -- in fact, almost identical -- to the Hebrew word for "believe" (amam), or faithful. Thus, it came to mean "sure" or "truly", an expression of absolute trust and confidence. -- HMM