I discussed this recently with some friends and different explanations regarding the words etymology were mentioned. I did some research and confirmed these two:
from Arab. aššāšīn "hashish eater" (pl.), to arab. ašīš "herbs, weeds, hemp plant, hashish", Member of a political-religious Islamic sect in the medieval. The sect's founder had his followers drink a hashed potion to make them compliant for the murders he ordered
from Italian - Assassinare, which literally means to kill someone. Furthermore it was mentioned by a friend that this word originated from latin and meant back then something like "throwing a stone at someone" - I could not find anything in this direction
Does anybody know which background is the original one?