Amên. In Hebrew is formed of the letters A M N = 1,40,50 =91,and is thus a simile of “Jehovah Adonai”=10, 5, 6, 5 and 1,4, 50,10 =91 together; it is one form of the Hebrew word for “truth”. In common parlance Amen is said to mean “so be it”. [ w.w.w.]
But, in esoteric parlance Amen means “the concealed”. Manetho Sebennites says the word signifies that which is hidden and we know through Hecatæus and others that the Egyptians used the word to call upon their great God of Mystery, Ammon (or “Ammas, the hidden god ”) to make himself conspicuous and manifest to them. Bonomi, the famous hieroglyphist, calls his worshippers very pertinently the “Amenoph”, and Mr. Bonwick quotes a writer who says: “Ammon, the hidden god, will remain for ever hidden till anthropomorphically revealed; gods who are afar off are useless”. Amen is styled “Lord of the new-moon festival”. Jehovah-Adonai is a new form of the ram-headed god Amoun or Ammon (q.v.) who was invoked by the Egyptian priests under the name of Amen.
Source: H.P.Blavatsky - The Theosophical Glossary