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un- +‎ fathom +‎ -able.


unfathomable (comparative more unfathomable, superlative most unfathomable)

  1. Impossible to fathom or understand.
    Synonyms: fathomless, incomprehensible, unintelligible; see also Thesaurus:incomprehensible
    Antonyms: explicable, intelligible; see also Thesaurus:comprehensible
    The sheer numbers they attacked with were unfathomable.
    • 2013 April 9, Andrei Lankov, “Stay Cool. Call North Korea’s Bluff.”, in New York Times[1]:
      Suggestions that those leaders are irrational and their decisions unfathomable are remarkably shallow. North Korea is not a theocracy led by zealots who preach the rewards of the afterlife.
  2. Difficult to penetrate.
    Synonyms: impenetrable, impregnable
    Antonyms: penetrable, pregnable

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