as ‧ sas ‧ sin /əˈsæsən, əˈsæsɪn/ BrE AmE noun [countable]
[ Date: 1500-1600 ; Language: Medieval Latin ; Origin: assassinus , from Arabic hashshashin 'one who smokes hashish (and then kills religious enemies)' , from hashish ]
someone who murders an important person:
Kennedy’s assassin is assumed to have been Lee Harvey Oswald.