Words are used in several cultures to mean different things. That dude named Amen was probably named that way, similar to people naming their kids April (month of the year), or Kunte (a famous person).
The word "amen" is a Hebrew word. The word is directly related -- in fact, almost identical -- to the Hebrew word for "believe" (amam), or faithful. Thus, it came to mean "sure" or "truly", an expression of absolute trust and confidence.
I like how it confirms God's promises to us...
Cursed be he that maketh the blind to wander out of the way. And all the people shall say, Amen. - Deuteronomy 27:18