The Collaborative International Dictionary of English v.0.48:

Unmentionable \Unmentionable\
   See mentionable.

WordNet (r) 3.0 (2006):

unmentionable
    adj 1: unsuitable or forbidden as a topic of conversation;
           "unmentionable words"
    n 1: a garment worn under other garments [syn: undergarment,
         unmentionable] [ant: outer garment, overgarment]

Moby Thesaurus II by Grady Ward, 1.0:

85 Moby Thesaurus words for "unmentionable":
   abject, abominable, appalling, arrant, atrocious, base, beggarly,
   cheesy, contemptible, crummy, debased, degraded, depraved,
   despicable, dirty, disgraceful, disgusting, dishonorable,
   execrable, filthy, flagrant, forbidden, foul, fulsome, grave,
   gross, heinous, immodest, incommunicable, indecent, indefinable,
   indescribable, ineffable, inenarrable, inexpressible, innominable,
   interdicted, lingerie, little, low, low-down, lumpen, mangy, mean,
   measly, miserable, monstrous, nefarious, noncommunicable,
   obnoxious, obscene, odious, paltry, petty, poky, poor, rank,
   reptilian, scabby, scandalous, scrubby, scruffy, scummy, scurvy,
   shabby, shameful, shocking, shoddy, skivvies, small, squalid,
   taboo, undefinable, underclothes, underclothing, undergarments,
   underwear, undies, unmentionables, unnameable, unspeakable,
   unutterable, unwhisperable, vile, wretched