assassin: [17] Etymologically, an assassin is an ‘eater or smoker of hashish’, the drug cannabis. In the Middle Ages, in the area of the Middle East and modern Iran, there was a sect of fanatical Ismaili Muslims, founded in the late 11th century by Hassan ibn Sabbah. Its members killed the sect’s opponents under the influence of cannabis. Hence the hashshāshīn (plural of hashshāsh, Arabic for ‘hashish-eater’) came to have a reputation as murderers. In English the Arabic plural form was perceived as singular. The word has retained its connotation of one who kills for political or religious rather than personal motives. => hashish
