Assassin

The word assassin is spelled this way - with four '-s-'s (and of course five in the plural assassins). The same is true of the related assassinate and assassination.

Etymological note: the word assassin comes from an Arabic word ځښٱښٻ hashīsh or ځښٱښي hashishi, meaning 'a user of hashish'. This word was applied by one sect (Mustali) of Shia Muslims to another, the Nizaris, and in Arabic was used as a term of political/religious abuse, roughly 'low-lifes', 'rabble', 'heretics' and so on. The particular group of Nizaris led by Hassan-i Sabbah (or Hassan bin Sabbah), which was based in Alamut (NW Persia, now Iran), adopted the political tactic of killing opposing leaders through individual highly trained hitmen. This tactic was employed against the invading Christians of the First Crusade.

Western xenophobia towards Muslims led to the development of a comprehensive narrative that told how 'The Old Man of the Mountains' (Hassan-i Sabbah) sent out suicidal killers to murder Christians. "The literal interpretation of this term in referring to the Nizaris (as hashish consuming intoxicated assassins) is rooted in the fantasies of medieval Westerners and their imaginative ignorance of Islam and the Ismailis" (Daftary, Farhad (1990) The Ismailis: Their history and doctrines, Cambridge, Cambridge University Press p 13, cited wikipedia). ({{OED]] (1885) reflects this: "Arabic ḥashshāshīn and ḥashīshiyyīn, plural of ḥashshāsh and ḥashīshiyy, lit[erally]. 'a hashish-eater, one addicted to hashish,' both forms being applied in Arabic to the Ismā[ayinold]īli sectarians, who used to intoxicate themselves with hashish or hemp, when preparing to dispatch some king or public man."

It is one of the oddities of etymology that assassin is a transliteration of the plural of the Arabic words. So the English plural assassins is really a double plural

This is one of the 117 mis-spellings listed as 'Common difficulties' in the section on 'Spelling' within 'Writing' in UEfAP.