The Collaborative International Dictionary of English v.0.48:

Unbearable \Unbearable\
   See bearable.

WordNet (r) 3.0 (2006):

unbearable
    adj 1: incapable of being put up with; "an intolerable degree of
           sentimentality" [syn: intolerable, unbearable,
           unendurable] [ant: tolerable]

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "unbearable":
   a bit much, absolute, arrant, classical, complete, consummate,
   crass, decided, definitive, downright, egregious, flagrant,
   glaring, gross, insufferable, insupportable, intolerable,
   out-and-out, outright, past bearing, perfect, positive, precious,
   profound, pronounced, proper, rank, regular, shattering, shocking,
   stark, stark-staring, superlative, surpassing, the veriest,
   thorough, thoroughgoing, too much, total, unacceptable,
   unconscionable, undeniable, unendurable, unequivocal, unmitigated,
   unqualified, unrelieved, unspoiled, unsupportable, utter