unmentionable

from 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}]

from The Collaborative International Dictionary of English v.0.48

Unmentionable \Unmentionable\
   See {mentionable}.

from 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