1530s (in Anglo-Latin from mid-13c.), via French and Italian, from Arabic hashishiyyin "hashish-users," plural of hashishiyy, from the source of hashish (q.v.).
A fanatical Ismaili Muslim sect of the mountains of Lebanon in 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), they had a reputation for murdering opposing leaders after intoxicating themselves by eating hashish. 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.