The Collaborative International Dictionary of English v.0.48:

bump off \bump` off"\, v. t.
   To kill (a person); to murder; -- used commonly of planned
   assassination by criminals; as, the mob bumped off all the
   witnesses. [Colloq.]
   [PJC]

WordNet (r) 3.0 (2006):

bump off
    v 1: kill intentionally and with premeditation; "The mafia boss
         ordered his enemies murdered" [syn: murder, slay,
         hit, dispatch, bump off, off, polish off,
         remove]