Mawson, C.O.S., ed. (1870–1938). Roget’s International Thesaurus. 1922.

Class IV. Words Relating to the Intellectual Faculties
Division (II) Communication of Ideas
Section I. Nature of Ideas Communicated

519. Unintelligibility.

   NOUN:UNINTELLIGIBILITY, incomprehensibility, imperspicuity [rare]; inconceivableness, unknowability, unknowableness, vagueness &c. adj.; obscurity; ambiguity [See Equivocalness]; doubtful meaning; uncertainty [See Uncertainty]; perplexity (confusion) [See Disorder]; spinosity; obscurum per obscurius [L.]; mystification (concealment) [See Concealment]; latency [See Latency]; transcendentalism.
  pons asinorum [L.], asses’ bridge; double Dutch, high Dutch [slang], Greek, Hebrew, Choctaw; jargon (unmeaning) [See Unmeaningness].
  ENIGMA, riddle (secret) [See Secret]; paradox; dignus vindice nodus [L.]; sealed book; steganography, cryptography, freemasonry.
   VERB:BE UNINTELLIGIBLE &c. adj.; require explanation [See Interpretation]; have a doubtful meaning, pass comprehension.
  RENDER UNINTELLIGIBLE &c. adj.; conceal [See Concealment]; darken [See Darkness]; confuse (derange) [See Derangement]; mystify, perplex (bewilder) [See Uncertainty].
  NOT UNDERSTAND [See Intelligibility]; lose, – the clew; miss; not know what to make of, be able to make nothing of, give it up; not be able to -account for, – make head or tail of; be at sea (uncertain) [See Uncertainty]; wonder [See Wonder]; see through a glass darkly (ignorance) [See Ignorance].
  not understand one another; play at cross-purposes (misinterpret) [See Misinterpretation].
   ADJECTIVE:UNINTELLIGIBLE, unaccountable, undecipherable, undiscoverable, unknowable, unfathomable; incognizable, inexplicable, inscrutable; inapprehensible, incomprehensible; insolvable, insoluble; impenetrable.
  PUZZLING, as Greek to one, unexplained, paradoxical, enigmatic or enigmatical, indecipherable, illegible.
  OBSCURE, crabbed, imperspicuous [rare], dark, muddy, clear as mud [colloq.], seen through a mist, dim, nebulous, shrouded in mystery; undiscernible (invisible) [See Invisibility]; misty (opaque) [See Opacity]; hidden [See Concealment]; latent [See Latency]; mysterious; mystic, mystical, acroamatic or acroamatical, metempiric or metempirical; transcendental; occult, esoteric, recondite, abstruse.
  INDEFINITE (indistinct) [See Invisibility]; perplexed (confused) [See Disorder]; undetermined, vague, loose, ambiguous.
  INCONCEIVABLE, inconceptible [obs.]; searchless; above -, beyond -, past- comprehension; beyond one’s depth; unconceived.
  INEXPRESSIBLE, unutterable, ineffable, undefinable, incommunicable.