You could try over at /r/etymology if nothing turns up here..

[Although I know that linking a website is not an appropriate answer but I will add that http://www.etymonline.com/ is a good source for answering this sort of question.]

It's from the Hebrew root ALEPH - MEM - NUN (אמן). The verbal form has the basic meaning "to be firm, trustworthy, safe." The verb is used e.g. in Genesis 42:20, where Joseph tells his brothers (who don't know him) that they need to leave the youngest brother "so that your words may be 'proved true' or 'verified.

AMEN is originally an adjective based on that verbal root. It meant 'trustworthy'. It was then used as an exclamation---something like "surely!"---in formal, solemn formulas where the hearer is accepting the validity of some statement (a curse, for example, or an exclamation), or accepts an order or announcement. The Jewish and Christian liturgical use comes from this form.

The root also has cognates in other semitic languages (Arabic, Aramaic, Phonecian).

Source:
Koehler, Baumgartner. The Hebrew and Aramaic Lexicon of the Old Testament, 2001.