WordNet (r) 3.0 (2006):

defeatist
    n 1: someone who is resigned to defeat without offering positive
         suggestions [syn: defeatist, negativist]

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "defeatist":
   Cassandra, Cassandra-like, Cassandran, Cassandrian, Eeyore,
   calamity howler, cynic, cynical, dismal, downbeat, gloomy, killjoy,
   malist, negative, negativist, negativistic, nihilist, nihilistic,
   pessimist, pessimistic, prophet of doom, retreatist, seek-sorrow,
   uncheerful, worrywart