From Wikipedia:
The Assassins (from Arabic: حشّاشين Ḥashshāshīn) were an order of Nizari Ismailis, particularly those of Persia and Syria that formed in the late 11th century. In time, the order began to pose a strong military threat to Sunni Seljuq authority within the Persian territories by capturing and inhabiting many mountain fortresses under the leadership of Hassan-i Sabbah.
The name "Assassin" is often said to derive from the Arabic Hashishin or "users of hashish", thought to have been originally derogatory and used by their adversaries during the Middle Ages. In actuality, the word is a misnomer for the Nizari Ismailis applied abusively to them by the Mustali Ismailis during the fall of the decaying Ismaili Fatimid Empire when the two streams separated from each other. In 1122 the Mustalian dynasty Fatimid caliph al-Amir referred to the Nizaris as the hashishiyya "without any explanation" and "without actually accusing them of using hashish, a product of hemp".
The modern word "assassin" originates from this Muslim order.
A popular myth says that the Assassins (part of them, at least) were killers who smoked hashish to suppress their conscience before proceeding to the killings.
I came across this story in several works of fiction, including La Casa dorada di Samarkand ("The Gilded House of Samarkand", from the Corto Maltese graphical novel series by Hugo Pratt), and more recently the Ghost in the Shell TV series. But after a tiny bit of research, it seems to be a fairly common tale.
Was part of it true, or it is only a myth?
