1530s (in Anglo-Latin from mid-13c.), via medieval French and Italian Assissini, Assassini, from Arabic hashīshīn "hashish-users," an Arabic nickname for the Nizari Ismaili sect in the Middle Easat during the Crusades, plural of hashishiyy, from the source of hashish (q.v.).
A fanatical Muslim sect in the mountains of Lebanon at the time of the Crusades, under leadership of the "Old Man of the Mountains" (which translates Arabic shaik-al-jibal, name applied to Hasan ibu-al-Sabbah), they had a reputation for murdering opposing leaders after intoxicating themselves by eating hashish. Their reputation has spread in Western Europe 12c.-13c.
The plural suffix -in was mistaken in Europe for part of the word (compare Bedouin). Middle English had the word as hassais (mid-14c.), from Old French hassasis, assasis, which is from the Arabic word.
The generalization of the sect's nickname to the meaning of any sort of assassin happened in Italian at the start of the 14th century. The word with the generalized meaning was often used in Italian in the 14th and 15th centuries. In the mid 16th century the generalized Italian word entered French, followed a little later by English. ["English Words of Arabic Ancestry"]