The Collaborative International Dictionary of English v.0.48:

Soothe \Soothe\ (s[=oo][th]), v. t. [imp. & p. p. Soothed; p.
   pr. & vb. n. Soothing.] [Originally, to assent to as true;
   OE. so[eth]ien to verify, AS. ges[=o][eth]ian to prove the
   truth of, to bear witness. See Sooth, a.]
   1. To assent to as true. [Obs.] --Testament of Love.
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   2. To assent to; to comply with; to gratify; to humor by
      compliance; to please with blandishments or soft words; to
      flatter.
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            Good, my lord, soothe him, let him take the fellow.
                                                  --Shak.
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            I've tried the force of every reason on him,
            Soothed and caressed, been angry, soothed again.
                                                  --Addison.
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   3. To assuage; to mollify; to calm; to comfort; as, to soothe
      a crying child; to soothe one's sorrows.
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            Music hath charms to soothe the savage breast,
            To soften rocks, or bend a knotted oak. --Congreve.
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            Though the sound of Fame
            May for a moment soothe, it can not slake
            The fever of vain longing.            --Byron.
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   Syn: To soften; assuage; allay; compose; mollify;
        tranquilize; pacify; mitigate.
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WordNet (r) 3.0 (2006):

soothe
    v 1: give moral or emotional strength to [syn: comfort,
         soothe, console, solace]
    2: cause to feel better; "the medicine soothes the pain of the
       inflammation" [ant: irritate]

Moby Thesaurus II by Grady Ward, 1.0:

80 Moby Thesaurus words for "soothe":
   abate, allay, alleviate, anesthetize, appease, assuage, balm,
   becalm, benumb, calm, calm down, cease, comfort, compose,
   conciliate, console, cool, cradle, cushion, deaden,
   deaden the pain, defuse, die down, diminish, dulcify, dull,
   dwindle, ease, ease matters, ebb, even out, foment, gentle,
   give relief, halt, hush, lay, lay the dust, lessen, lull, mitigate,
   molder, mollify, numb, pacify, pad, palliate, placate, poultice,
   pour balm into, pour balm on, pour oil on, propitiate, quell,
   quiesce, quiet, quieten, reduce, relieve, rest, rock,
   rock to sleep, salve, settle, slacken, slake, smooth, smooth down,
   smooth over, smoothen, soften, stabilize, steady, still, stop,
   stupe, subdue, subside, tranquilize, wane