The Collaborative International Dictionary of English v.0.48:

Pessimist \Pes"si*mist\, n. [L. pessimus worst: cf. F.
   pessimiste.]
   1. (Metaph.) One who advocates the doctrine of pessimism; --
      opposed to optimist.
      [1913 Webster]

   2. One who looks on the dark side of things.
      [1913 Webster] Pessimist

The Collaborative International Dictionary of English v.0.48:

Pessimist \Pes"si*mist\, Pessimistic \Pes`si*mis"tic\, a.
   (Metaph.)
   Of or pertaining to pessimism; characterized by pessimism;
   gloomy; foreboding. "Giving utterance to pessimistic doubt."
   --Encyc. Brit.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pessimist
    n 1: a person who expects the worst [ant: optimist]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "pessimist":
   Cassandra, Eeyore, calamity howler, crepehanger, cynic, damp,
   damper, defeatist, fussbudget, killjoy, malist, misanthrope,
   negativist, nihilist, prophet of doom, retreatist, seek-sorrow,
   spoilsport, wet blanket, worrywart