In ancient Egypt, Amn or Amun was the Deity of the Upper Nile. The name was revered as far as
ancient Rome, where Egyptian temples where highly regarded. Some Egyptian Pharoahs were named after
Amn, e.g. Tutankhamen and Amenhotep, similar to the Yoruba style of naming after the God of thunder and lightning; Sango: Sangowawa, Sangotosun.

It may shock some of us that the whole world is trying to invoke an African God when the say Amen or Amin.

I've heard of this theory before but don't really believe it.
A better explanation is that the word comes from the Hebrew "aman", which means to be firm or solid in the sense of permanency. Thus by implication, it means to be sure, true or faithful. So whenever we see this word Amen used in scripture, it is affirming truth, or illustrating something is said that is of absolute certainty.

http://en.wikipedia.org/wiki/Amen

That said, it is possible that the root of the word is based in ancient Egyptian religion. After all, the Hebrews did originally come from there in Exodus.

more like a Semitic term, imo.