The Collaborative International Dictionary of English v.0.48:

Unhappy \Un*hap"py\, a.
   1. Not happy or fortunate; unfortunate; unlucky; as, affairs
      have taken an unhappy turn.
      [1913 Webster]

   2. In a degree miserable or wretched; not happy; sad;
      sorrowful; as, children render their parents unhappy by
      misconduct.
      [1913 Webster]

   3. Marked by infelicity; evil; calamitous; as, an unhappy
      day. "The unhappy morn." --Milton.
      [1913 Webster]

   4. Mischievous; wanton; wicked. [Obs.] --Shak.
      [1913 Webster] -- Un*hap"pi*ly, adv. --
      Un*hap"pi*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unhappiness
    n 1: emotions experienced when not in a state of well-being
         [syn: sadness, unhappiness] [ant: happiness]
    2: state characterized by emotions ranging from mild
       discontentment to deep grief [ant: felicity, happiness]

Moby Thesaurus II by Grady Ward, 1.0:

99 Moby Thesaurus words for "unhappiness":
   Faustianism, angst, anguish, anxiety, ban, blackball, blackballing,
   blues, boredom, cheerlessness, cold comfort, complaint, dejection,
   depression, dim view, disagreement, disappointment, disapprobation,
   disapproval, discomfort, discomposure, discontent,
   discontentedness, discontentment, disenchantment, disesteem,
   disfavor, disgruntlement, disillusion, disillusionment, dislike,
   displeasure, disquiet, disrespect, dissatisfaction,
   dissatisfiedness, dissent, distaste, divine discontent, dread,
   dullness, emptiness, ennui, envy, exclusion, existential woe,
   flatness, grimness, humorlessness, ill humor, indignation,
   infelicity, infestivity, inquietude, joylessness, lack of pleasure,
   low estimation, low opinion, malaise, mirthlessness, misery,
   nausea, nongratification, nonsatisfaction, objection, opposition,
   opposure, ostracism, painfulness, peevishness, petulance, protest,
   querulousness, rebelliousness, rejection, resentment, restiveness,
   restlessness, savorlessness, sourness, spleen, staleness,
   sulkiness, tastelessness, tediousness, tedium, thumbs-down,
   uncheerfulness, uncomfortableness, unease, uneasiness,
   unfulfillment, unjoyfulness, unmirthfulness, unpleasure,
   unsatisfaction, vexation of spirit, woe, wretchedness