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
assassin (n.)
1530s (in Anglo-Latin from mid-13c.), via French and Italian, from Arabic hashishiyyin "hashish-users," plural of hashishiyy, from the source of hashish (q.v.). A fanatical Ismaili Muslim sect of the time of the Crusades, under leadership of the "Old Man of the Mountains" (translates Arabic shaik-al-jibal, name applied to Hasan ibu-al-Sabbah), with a reputation for murdering opposing leaders after intoxicating themselves by eating hashish. The plural suffix -in was mistaken in Europe for part of the word (compare Bedouin).