The Collaborative International Dictionary of English v.0.48:

Sincere \Sin*cere"\, a. [Compar. Sincerer; superl.
   Sincerest.] [L. sincerus, of uncertain origin; the first
   part perhaps akin to sin- in singuli (see Single), and the
   second to cernere to separate (cf. Discern): cf. F.
   sinc[`e]re.]
   1. Pure; unmixed; unadulterated.
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            There is no sincere acid in any animal juice.
                                                  --Arbuthnot.
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            A joy which never was sincere till now. --Dryden.
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   2. Whole; perfect; unhurt; uninjured. [Obs.]
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            The inviolable body stood sincere.    --Dryden.
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   3. Being in reality what it appears to be; having a character
      which corresponds with the appearance; not falsely
      assumed; genuine; true; real; as, a sincere desire for
      knowledge; a sincere contempt for meanness.
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            A sincere intention of pleasing God in all our
            actions.                              --Law.
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   4. Honest; free from hypocrisy or dissimulation; as, a
      sincere friend; a sincere person.
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            The more sincere you are, the better it will fare
            with you at the great day of account. --Waterland.
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   Syn: Honest; unfeigned; unvarnished; real; true; unaffected;
        inartificial; frank; upright. See Hearty.
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WordNet (r) 3.0 (2006):

sincere
    adj 1: open and genuine; not deceitful; "he was a good man,
           decent and sincere"; "felt sincere regret that they were
           leaving"; "sincere friendship" [ant: insincere]
    2: characterized by a firm and humorless belief in the validity
       of your opinions; "both sides were deeply in earnest, even
       passionate"; "an entirely sincere and cruel tyrant"; "a film
       with a solemn social message" [syn: earnest, sincere,
       solemn]

Moby Thesaurus II by Grady Ward, 1.0:

174 Moby Thesaurus words for "sincere":
   abandoned, aboveboard, actual, afire, ardent, artless, authentic,
   bluff, blunt, bona fide, born yesterday, bound,
   bound and determined, broad, brusque, burning, candid,
   card-carrying, childlike, committed, confiding, dear, decided,
   decisive, dedicated, definite, determined, devoted, devout, dinkum,
   direct, downright, earnest, explicit, faithful, fervent, fervid,
   fiery, flaming, following the letter, forthright, frank,
   frankhearted, free, free-speaking, free-spoken, free-tongued,
   genuine, good, guileless, heart-to-heart, heartfelt, hearty,
   heated, honest, honest-to-God, hot, hot-blooded, impassioned,
   in earnest, inartificial, ingenu, ingenuous, innocent, intense,
   intent, intent on, lawful, legitimate, lifelike, literal, loyal,
   meant, naive, natural, naturalistic, obstinate, on fire,
   on the level, open, openhearted, original, outspoken, passionate,
   perfervid, persevering, persistent, plain, plain-spoken, pure,
   purposeful, real, realistic, red-hot, relentless, resolute,
   resolved, rightful, round, serious, simon-pure, simple,
   simplehearted, simpleminded, single-hearted, single-minded,
   spirited, sterling, straight, straight-out, straightforward,
   sure-enough, tenacious, transparent, true, true to life,
   true to nature, true to reality, trustful, trusting, truthful,
   unadulterated, unaffected, unassumed, unassuming, unchecked,
   uncolored, unconcocted, unconstrained, uncopied, uncounterfeited,
   undesigning, undisguised, undisguising, undissembled,
   undissembling, undistorted, unequivocal, unexaggerated,
   unfabricated, unfanciful, unfeigned, unfeigning, unfictitious,
   unflattering, unguarded, unimagined, unimitated, uninvented,
   unpretended, unpretending, unpretentious, unqualified, unreserved,
   unrestrained, unromantic, unsimulated, unsophisticated, unspecious,
   unsuspicious, unsynthetic, unvarnished, unwary, vehement,
   veracious, verbal, verbatim, veridical, verisimilar, warm,
   white-hot, wholehearted, word-for-word, zealous