The Collaborative International Dictionary of English v.0.48:

Opaque \O*paque"\, n.
   That which is opaque; opacity. --Young.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Opaque \O*paque"\, a. [F., fr. L. opacus. Cf. Opacous.]
   [1913 Webster]
   1. Impervious to the rays of light; not transparent; as, an
      opaque substance.
      [1913 Webster]

   2. Obscure; not clear; unintelligible. [Colloq.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

opaque
    adj 1: not transmitting or reflecting light or radiant energy;
           impenetrable to sight; "opaque windows of the jail";
           "opaque to X-rays" [ant: clear]
    2: not clearly understood or expressed [syn: opaque,
       unintelligible]

Moby Thesaurus II by Grady Ward, 1.0:

97 Moby Thesaurus words for "opaque":
   Boeotian, abstruse, adiaphanous, ambiguous, amorphous,
   amphibological, arcane, asinine, backward, baffling, becloud,
   beef-brained, beef-witted, black, blind, blockish, blurred, blurry,
   bovine, chumpish, clear as mud, cloddish, cloud, clouded, cloudy,
   cowish, crass, cryptic, dark, darken, dense, dim, doltish, dull,
   dull-witted, dullard, dumb, duncical, duncish, dunderheaded,
   elusive, enigmatic, equivocal, fat, foggy, fuzzy, gross, grumly,
   hazy, impenetrable, impermeable, impervious to light, indefinite,
   indeterminate, indistinct, ineducable, intransparent, klutzy,
   lumpish, misty, muddy, murky, mysterious, mystifying, nebulous,
   nubilous, oafish, obfuscated, obscure, obscured, obtuse,
   perplexing, puzzling, recondite, roiled, roily, shadowy, shapeless,
   slow, smoky, sottish, stolid, stupid, tenebrous, thick,
   thickheaded, transcendent, turbid, uncertain, unclear,
   unfathomable, unintelligent, unintelligible, unplain, unteachable,
   vague, wrongheaded