Even seminarians might struggle a bit with this one. For example, those of us who've taken Greek are taught that the word (ἀμήν) can be translated "truly," as in the phrase "Truly, truly, I say to you." As it happens, the word isn't actually a Greek word, but a Hebrew one (although since אָמֵן appears in only 24 verses in the Hebrew scriptures, as opposed to the Greek version's 126 verses in the New Testament, students who only took entry-level Hebrew, as I did, might be forgiven for not catching the word as being Hebrew). As anyone who knows about how translation works might guess, "truly" doesn't capture the full range of the word's use. It also expresses agreement and signifies reliability. To end a prayer with "amen" is to indicate that you wish to affirm the statements included in the prayer as being a true account of your intentions before God.
According to one theory, the concept behind the word didn't actually originate with the Hebrews, either. Perhaps ironically, the word would seem to have its origins in Egypt, attested to around 2500 BC. This would be early enough to predate the stories of Moses (variously dated from the 18th to the 7th century BC, depending on who you're talking to*), where the word first appears. (NOTE: Even if one accepts that a similar word which appears in the story of Abraham is indeed related to this one, it should be noted that tradition says that Moses was the one who wrote those words down. While many Christians think that these passages were written down later than Moses, pretty much no one suggests an earlier scribe!) As Charles Panati writes in Panati's Extraordinary Origins of Everyday Things:
To the Egyptians, Amun meant "the hidden one" and was the name of their highest deity.... As later cultures invoked the god Jupiter with the exclamation "By Jove!" the Egyptians called on their deity: "By Amun!"A strong argument against this theory would be the fact that calling on the name of a foreign God would, of course, be anathema to Jews and Christians alike, leaving one to ask how such a situation could ever have been allowed to occur. My sources are silent on this question, but it should be noted that the Jewish people had lived in Egypt for several hundred years by the time God spoke to Moses. Prior to this time, there is little indication that the Jews even knew who God was in a concrete sense, and the Commandment prohibiting worship of other gods was certainly not yet given to the people. Thus, it isn't so hard to imagine how a phrase common in the land in which one is living can be innocently assimilated into the culture, and it's core meaning shifted from an outright reference to a foreign god to a statement affirming truth. Obviously, anyone who would invoke the name of God on something would be attesting to the truth of that thing! In such a scenario, by the time the Jewish people learned about their God when they were rescued from Egypt, and certainly before the injunctions against invoking other gods were commanded, the linguistic shift had already taken place.
Scandalous? Sure. Provable? Probably not. Indeed, Wikipedia notes that "there is no academic support for" this view. But I got you to read the entry, didn't I? And now you know more about the definition of "amen," don't you? ;)
*That's not a typo. There really is a difference of opinion spanning more than thousand years in regard to the historical origins of Moses.