The Collaborative International Dictionary of English v.0.48:

Outsider \Out`sid"er\, n.
   1. One not belonging to the concern, institution, party,
      etc., spoken of; one disconnected in interest or feeling.
      [Recent] --A. Trollope.
      [1913 Webster]

   2. A locksmith's pinchers for grasping the point of a key in
      the keyhole, to open a door from the outside when the key
      is inside.
      [1913 Webster]

   3. A horse which is not a favorite in the betting. [Cant]
      [1913 Webster]

WordNet (r) 3.0 (2006):

outsider
    n 1: someone who is excluded from or is not a member of a group
         [syn: foreigner, outsider]
    2: a contestant (human or animal) not considered to have a good
       chance to win

Moby Thesaurus II by Grady Ward, 1.0:

82 Moby Thesaurus words for "outsider":
   Jim Crow, Uitlander, alien, apartheid, auslander, barbarian, case,
   character, color bar, crackpot, crank, deracine, displaced person,
   division, eccentric, emigre, ethnocentrism, exclusiveness, exile,
   fanatic, foreign devil, foreigner, gatecrasher, gringo, guest,
   hermit, hobo, insularity, insulation, interloper, intruder,
   invader, isolation, know-nothingism, kook, lone wolf, loner,
   maverick, meshuggenah, narrowness, natural, newcomer,
   nonconformist, nut, odd fellow, oddball, oddity, original,
   out-group, outcast, outlander, outlaw, pariah, parochialism,
   persona non grata, quarantine, queer duck, queer fish,
   queer specimen, race hatred, racial segregation, rara avis,
   refugee, screwball, seclusion, segregation, separation,
   snobbishness, solitary, squatter, stranger, the Wandering Jew,
   tightness, tramontane, tramp, trespasser, type, ultramontane,
   visitor, wanderer, xenophobia, zealot