a murderer, especially one who kills a politically prominent person for fanatical or monetary reasons.
2.
(initial capital letter) one of an order of Muslim fanatics, active in Persia and Syria from about 1090 to 1272, whose chief object was to assassinate Crusaders.
Origin: 1525–35; < Medieval Latin assassinī (plural) < Arabic ḥashshāshīn eaters of hashish
1530s (in Anglo-L. from mid-13c.), via Fr. and It., from Arabic hashishiyyin "hashish-users," pl. of hashishiyy, from 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 pl. suffix -in was mistaken in Europe for part of the word (cf. Bedouin).