The Collaborative International Dictionary of English v.0.48:

Spleen \Spleen\, v. t.
   To dislke. [Obs.] --Bp. Hacket.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Spleen \Spleen\, n. [L. splen, Gr. ???? the milt or spleen,
   affection of the spleen; cf. L. lien, plihan, pl[imac]han.]
   1. (Anat.) A peculiar glandlike but ductless organ found near
      the stomach or intestine of most vertebrates and connected
      with the vascular system; the milt. Its exact function in
      not known.
      [1913 Webster]

   2. Anger; latent spite; ill humor; malice; as, to vent one's
      spleen.
      [1913 Webster]

            In noble minds some dregs remain,
            Not yet purged off, of spleen and sour disdain.
                                                  --Pope.
      [1913 Webster]

   3. A fit of anger; choler. --Shak.
      [1913 Webster]

   4. A sudden motion or action; a fit; a freak; a whim. [Obs.
      or R.]
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            A thousand spleens bear her a thousand ways. --Shak.
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   5. Melancholy; hypochondriacal affections.
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            Bodies changed to various forms by spleen. --Pope.
      [1913 Webster]

            There is a luxury in self-dispraise:
            And inward self-disparagement affords
            To meditative spleen a grateful feast. --Wordsworth.
      [1913 Webster]

   6. A fit of immoderate laughter or merriment. [Obs.]
      [1913 Webster]

            Thy silly thought enforces my spleen. --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

spleen
    n 1: a large dark-red oval organ on the left side of the body
         between the stomach and the diaphragm; produces cells
         involved in immune responses [syn: spleen, lien]
    2: a feeling of resentful anger [syn: irascibility, short
       temper, spleen, quick temper]

Moby Thesaurus II by Grady Ward, 1.0:

131 Moby Thesaurus words for "spleen":
   abdomen, acerbity, acid, acidity, acidulousness, acrimony, anger,
   angst, anguish, animosity, anus, anxiety, appendix, asperity,
   bad humor, bad temper, bile, biliousness, bitter resentment,
   bitterness, bitterness of spirit, blind gut, boredness, boredom,
   bowels, brain, causticity, cecum, cheerlessness, choler, colon,
   corrosiveness, despite, despitefulness, discomfort, discomposure,
   discontent, dislike, dispiritedness, displeasure, disquiet,
   dissatisfaction, dread, dullness, duodenum, emptiness, endocardium,
   ennui, entrails, existential woe, fed-upness, flatness, foregut,
   gall, giblets, gizzard, gnashing of teeth, grimness, grudge, guts,
   hard feelings, heart, heartburning, hindgut, ill, ill humor,
   ill nature, ill temper, innards, inner mechanism, inquietude,
   insides, internals, intestine, inwards, jadedness, jejunum,
   joylessness, kidney, kishkes, lack of pleasure, large intestine,
   life-weariness, liver, liver and lights, lung, malaise, melancholy,
   midgut, nausea, nongratification, nonsatisfaction, painfulness,
   perineum, pump, pylorus, rancor, rankling, rectum, satiation,
   satiety, savorlessness, slow burn, small intestine, soreness,
   sourness, staleness, stomach, taedium vitae, tastelessness,
   tediousness, tedium, ticker, tiredness, tripes, uncomfortableness,
   unease, uneasiness, unhappiness, unpleasure, unsatisfaction,
   vermiform appendix, vexation of spirit, virulence, viscera, vitals,
   wearifulness, weariness, works, world-weariness, wrath