Thayer's Greek Lexicon

 1. firm
  1) metaph. faithful
2. verily, amen
  1) at the beginning of a discourse - surely, truly, of a
truth
  2) 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.

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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