amen: [OE] Amen was originally a Hebrew noun, āmēn ‘truth’ (based on the verb āman ‘strengthen, confirm’), which was used adverbially as an expression of confirmation or agreement. Biblical texts translated from Hebrew simply took it over unaltered (the Greek Septuagint has it, for example), and although at first Old English versions of the gospels substituted an indigenous term, ‘truly’, by the 11th century amen had entered English too.
Old English, from Late Latin amen, from Ecclesiastical Greek amen, from Hebrew amen "truth," used adverbially as an expression of agreement (as in Deut. xxvii:26, I Kings i:36; compare Modern English verily, surely, absolutely in the same sense), from Semitic root a-m-n "to be trustworthy, confirm, support." Used in Old English only at the end of Gospels, otherwise translated as Soðlic! or Swa hit ys, or Sy! As an expression of concurrence after prayers, it is recorded from early 13c.