The Collaborative International Dictionary of English v.0.48:

Unintelligible \Unintelligible\
   See intelligible.

WordNet (r) 3.0 (2006):

unintelligible
    adj 1: poorly articulated or enunciated, or drowned by noise;
           "unintelligible speech" [ant: intelligible]
    2: not clearly understood or expressed [syn: opaque,
       unintelligible]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "unintelligible":
   Greek to one, ambiguous, amphibological, beyond understanding,
   equivocal, impenetrable, inarticulate, incognizable, incoherent,
   incomprehensible, inscrutable, numinous, opaque,
   past comprehension, rambling, tenebrous, uncertain, unclear,
   unconnected, unfathomable, unknowable, unsearchable, vague



Bouvier's Law Dictionary, Revised 6th Ed (1856):

UNINTELLIGIBLE. That which cannot be understood.
     2. When a law, a contract, or will, is unintelligible, it has no effect
whatever. Vide Construction, and the authorities there referred to.