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]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "bump off":
   assassinate, blast, blot out, cool, croak, do in, dust off, erase,
   execute, finish, fix, get, give the business, gun down, hit, ice,
   knock off, lay out, liquidate, off, polish off, put away, rub out,
   settle, take care of, waste, wipe out, zap