Source : Webster's Revised Unabridged Dictionary (1913)

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

   2. One who looks on the dark side of things.

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.

Source : WordNet®

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