The Collaborative International Dictionary of English v.0.48:

jerk off \jerk` off"\, v. i.
   To masturbate. [vulgar slang]

   Syn: whack off.
        [PJC]

WordNet (r) 3.0 (2006):

jerk off
    v 1: get sexual gratification through self-stimulation [syn:
         masturbate, wank, fuck off, she-bop, jack off,
         jerk off]