The Collaborative International Dictionary of English v.0.48:

Insincere \In`sin*cere"\, a. [L. insincerus. See In- not, and
   Sincere.]
   [1913 Webster]
   1. Not being in truth what one appears to be; not sincere;
      dissembling; hypocritical; disingenuous; deceitful; false;
      -- said of persons; also of speech, thought; etc.; as,
      insincere declarations.
      [1913 Webster]

   2. Disappointing; imperfect; unsound. [Obs.]
      [1913 Webster]

            To render sleep's soft blessings insincere. --Pope.

   Syn: Dissembling; hollow; hypocritical; deceptive deceitful;
        false; disingenuous; untrustworthy.
        [1913 Webster]

WordNet (r) 3.0 (2006):

insincere
    adj 1: lacking sincerity; "a charming but thoroughly insincere
           woman"; "their praise was extravagant and insincere"
           [ant: sincere]

Moby Thesaurus II by Grady Ward, 1.0:

118 Moby Thesaurus words for "insincere":
   Machiavellian, Tartuffian, Tartuffish, adulatory, affected,
   ambidextrous, apparently sound, artful, artificial, bland,
   blandishing, blarneying, buttery, cajoling, calculating, canting,
   casuistic, colorable, complimentary, courtierly, courtly, crafty,
   crooked, cunning, deceitful, deceptive, dishonest, disingenuous,
   dissembling, double, double-dealing, duplicitous, empty, evasive,
   fair-spoken, faithless, fallacious, false, falsehearted, fawning,
   fine-spoken, flattering, forsworn, foxy, fulsome, goody,
   goody-goody, gushing, histrionic, holier-than-thou, hollow,
   honey-mouthed, honey-tongued, honeyed, hypocritical, illusive,
   insinuating, jesuitic, la-di-da, lying, maniere, mannered,
   mealymouthed, mendacious, obsequious, oily, oily-tongued,
   overacted, overdone, overrefined, oversubtle, perfidious, perjured,
   pharisaic, philosophistic, pietistic, pious, plausible,
   pretentious, sanctified, sanctimonious, scheming, self-righteous,
   shifty, slick, slimy, slippery, slobbery, sly, smarmy, smooth,
   smooth-spoken, smooth-tongued, sniveling, soapy, soft-soaping,
   sophistic, sophistical, specious, stagy, sycophantic, theatrical,
   tongue in cheek, treacherous, tricky, two-faced, uncandid,
   unctuous, underhanded, unfrank, ungenuine, unnatural, unserious,
   unsincere, untruthful, vulpine, wheedling, wily