The Collaborative International Dictionary of English v.0.48:

Amen \A`men"\, v. t.
   To say Amen to; to sanction fully.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Amen \A`men"\ (?; 277), interj., adv., & n. [L. amen, Gr.
   'amh`n, Heb. [=a]m[=e]n certainly, truly.]
   An expression used at the end of prayers, and meaning, So be
   it. At the end of a creed, it is a solemn asseveration of
   belief. When it introduces a declaration, it is equivalent to
   truly, verily.

   Note: It is used as a noun, to denote:
   (a) concurrence in belief, or in a statement; assent;
   (b) the final word or act;
   (c) Christ as being one who is true and faithful.
       [1913 Webster]

             And let all the people say, Amen.    --Ps. cvi. 48.
       [1913 Webster]

             Amen, amen, I say to thee, except a man be born
             again, he can not see the kingdom of God. --John
                                                  ii. 3. Rhemish
                                                  Trans.
       [1913 Webster]

   To say amen to, to approve warmly; to concur in heartily or
      emphatically; to ratify; as, I say Amen to all.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Amen
    n 1: a primeval Egyptian personification of air and breath;
         worshipped especially at Thebes [syn: Amen, Amon,
         Amun]

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "amen":
   OK, Roger, absolutely, affirmative, affirmative attitude,
   affirmativeness, agreed, all right, alright, alrighty, as you say,
   assuredly, aye, be it so, by all means, certainly, da, done,
   exactly, fine, good, good enough, hear, indeed, indeedy,
   it is that, ja, just so, mais oui, most assuredly, naturally,
   naturellement, nod, nod of assent, of course, okay, oui,
   positively, precisely, quite, rather, really, right, right as rain,
   right you are, righto, so be it, so is it, so it is, sure,
   sure thing, surely, that is so, thumbs-up, to be sure, truly,
   very well, well and good, why yes, yea, yea-saying, yeah, yep, yes,
   yes indeed, yes indeedy, yes sir, yes sirree, you are right,
   you bet, you said it, you speak truly



Easton's 1897 Bible Dictionary:

Amen
   This Hebrew word means firm, and hence also faithful (Rev.
   3:14). In Isa. 65:16, the Authorized Version has "the God of
   truth," which in Hebrew is "the God of Amen." It is frequently
   used by our Saviour to give emphasis to his words, where it is
   translated "verily." Sometimes, only, however, in John's Gospel,
   it is repeated, "Verily, verily." It is used as an epithet of
   the Lord Jesus Christ (Rev. 3:14).

     It is found singly and sometimes doubly at the end of prayers
   (Ps. 41:13; 72:19; 89:52), to confirm the words and invoke the
   fulfilment of them. It is used in token of being bound by an
   oath (Num. 5:22; Deut. 27:15-26; Neh. 5:13; 8:6; 1 Chr. 16:36).
   In the primitive churches it was common for the general audience
   to say "Amen" at the close of the prayer (1 Cor. 14:16).

     The promises of God are Amen; i.e., they are all true and sure
   (2 Cor. 1:20).