The term “assassin” likely has roots in hashshāshīn (“hashish smokers or users”), a mispronunciation of the original Asāsiyyūn, but not a mispronunciation of Assasiyeen (pronounced “Asāsiyyeen”, the plural of “Asasi”). Originally referring to the methods of political control exercised by the Assasiyuun, one can see how it became “assassin” in several languages to describe similar activities anywhere.

Apparently by smoking.