unverifiable

/ʌnˈvɛrəˌfaɪəbəl/
adjective
  1. Impossible to check or prove as true or accurate.
    • Many claims made in online advertisements are unverifiable without purchasing the product.
    • Historians often struggle with unverifiable details in ancient manuscripts.
    • The witness gave an unverifiable account of the event, with no one else present to confirm it.