The English word “assassins” was originally used to denote a group of fanatical Ismailis (a Shi’ite Muslim sect) who, between 1094 and 1273, worked for the creation of a new Fatimid caliphate, murdering prominent individuals. They murdered prominent individuals; hence, “assassin” in English came to mean a politically motivated murderer.
The name derives from the Arabic “hashashin” — Marco Polo and other European chroniclers claimed that the Assassins used hashish to stimulate their fearless acts. For example, Brewer writes:
“Assassins. A band of Carmathians, collected by Hassa, subah of Nishapour, called the Old Man of the Mountains, because he made Mount Lebanon his stronghold. This band was the terror of the world for two centuries, when it was put down by Sultan Bibaris. The assassins indulged in haschisch (bang), an intoxicating drink, and from this liquor received their name.”
For more information, see also the Hawkwind song ‘Hassan I Sabbah’ on their album Quark, Strangeness and Charm.