Roget's Thesaurus of English Words and Phrases by Peter Mark Roget

imperspicuity†; inconceivableness, vagueness &c adj.; obscurity; ambiguity &c 520; doubtful meaning; uncertainty &c 475; perplexity &c (confusion) 59; spinosity†; obscurum per obscurius [Lat.]; mystification &c (concealment) 528; latency &c 526; transcendentalism. paradox, oxymoron; riddle, enigma, puzzle &c (secret) 533; diagnus vindice nodus [Lat.]; sealed book; steganography†, freemasonry. pons asinorum [Lat.], asses' bridge; high Dutch, Greek, Hebrew; jargon &c (unmeaning) 517. V. be unintelligible &c adj.; require explanation &c 522; have a doubtful meaning, pass comprehension. render unintelligible &c adj.; conceal &c 528; darken &c 421; confuse &c (derange) 61; perplex &c (bewilder) 475. not understand &c 518; lose, lose the clue; miss; not know what to make of, be able to make nothing of, give it up; not be able to account for, not be able to make either head or tail of; be at sea &c (uncertain) 475; wonder &c 870; see through a glass darkly &c (ignorance) 491. not understand one another; play at cross purposes &c (misinterpret) 523. Adj. unintelligible, unaccountable, undecipherable, undiscoverable, unknowable, unfathomable; incognizable†, inexplicable, inscrutable; inapprehensible†, incomprehensible; insolvable†, insoluble; impenetrable. illegible, as Greek to one, unexplained, paradoxical; enigmatic, enigmatical, puzzling (secret) 533; indecipherable. obscure, dark, muddy, clear as mud, seen through a mist, dim, nebulous, shrouded in mystery; opaque, dense; undiscernible &c (invisible) 447 [Obs.]; misty &c (opaque) 426; hidden &c 528; latent &c 526. indefinite, garbled &c (indistinct) 447; perplexed &c (confused) 59; undetermined, vague, loose, ambiguous; mysterious; mystic, mystical; acroamatic†, acroamatical†; metempirical; transcendental; occult, recondite, abstruse, crabbed. inconceivable, inconceptible†; searchless†; above comprehension, beyond comprehension, past comprehension; beyond one's depth; unconceived. inexpressible, undefinable, incommunicable. unpredictable, unforeseeable. Phr. it's Greek to me.