WordNet (r) 3.0 (2006):

pop off
    v 1: leave quickly
    2: pass from physical life and lose all bodily attributes and
       functions necessary to sustain life; "She died from cancer";
       "The children perished in the fire"; "The patient went
       peacefully"; "The old guy kicked the bucket at the age of
       102" [syn: die, decease, perish, go, exit, pass
       away, expire, pass, kick the bucket, cash in one's
       chips, buy the farm, conk, give-up the ghost, drop
       dead, pop off, choke, croak, snuff it] [ant: be
       born]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "pop off":
   blow, buy it, cash in, check in, check out, croak, die, drop off,
   exit, get away, get off, go, go west, have it, kick in, kick off,
   kick the bucket, knock off, leave, pass, pass away, pass out,
   peg out, perish, pipe off, pull out, retire, run along, shove off,
   step off, succumb, take off