'assassinatress' originates from English, specifically formed from the noun 'assassin' plus the feminine suffix '-tress' (from Old French '-tresse'), where '-tress' meant 'female agent' or 'female doer'.

'assassin' originally came into European languages from Arabic 'ḥashshāshīn' (often rendered in Medieval Latin and Old French), and later English formed feminine variants by adding the suffix '-tress' to create words like 'assassinatress'.

Initially 'assassin' referred to members of a particular sect ('hashish-users') and later came to mean a person who murders for political or other motives; 'assassinatress' specifically extended that sense to indicate a female assassin.