WordNet (r) 3.0 (2006):

lifer
    n 1: a prisoner serving a term of life imprisonment

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "lifer":
   POW, cageling, captive, chain gang, con, convict, detenu,
   ex-convict, gaolbird, internee, jailbird, parolee,
   political prisoner, prisoner, prisoner of war, stir bird,
   ticket-of-leave man, ticket-of-leaver, trusty