Source : Webster's Revised Unabridged Dictionary (1913)

Assassin \As*sas"sin\, n. [F. (cf. It. assassino), fr. Ar.
   `hashishin one who has drunk of the hashish. Under its
   influence the Assassins of the East, followers of the Shaikh
   al-Jabal (Old Man of the Mountain), were said to commit the
   murders required by their chief.]
   One who kills, or attempts to kill, by surprise or secret
   assault; one who treacherously murders any one unprepared for
   defense.

Assassin \As*sas"sin\, v. t.
   To assassinate. [Obs.] --Stillingfleet.

Source : WordNet®

assassin
     n 1: a murderer (especially one who kills a prominent political
          figure) who kills by a treacherous surprise attack and
          often is hired to do the deed; "his assassins were
          hunted down like animals"; "assassinators of kings and
          emperors" [syn: {assassinator}, {bravo}]
     2: a member of a secret order of Muslims (founded in the 12th
        century) who terroriszed and killed Christian Crusaders