1. A killer, especially of a political leader or other public figure.
2. Someone who murders by surprise attack; especially, a person who carries out a plot to kill a prominent person.
3. A member of a secret order of Muslim fanatics (founded in the 12th century), active in Persia and Syria from about 1090 to 1272, whose primary object was to assassinate, terrorize, and to kill Christian Crusaders.
4. Etymology: in Anglo-Latin from about the mid 13th century, via French and Italian, from Arabic hashsashin, "hashish-users", a plural of hashshash, Arabic for "hashish-eater".

In English, the Arabic plural form was thought to be a singular form which became known as a fanatical Ismaili Muslim sect at the time of the Crusades known to murder opposing leaders after intoxicating themselves by eating or smoking hashish or the drug cannabis.