undoubtable

From Middle English undoutable, undoutabil, equivalent to un- +‎ doubtable.

undoubtable (not comparable)

1. (rare) Incapable of being doubted; undoubted; indubitable.
   • 1904, John Galsworthy, chapter 20, in The Island Pharisees:

     Her undoubtable quality had a non-individual flavour, as if standing for her class.

• undoubtably

• “undoubtable”, in Webster’s Revised Unabridged Dictionary, Springfield, Mass.: G. & C. Merriam, 1913, →OCLC.
• “undoubtable”, in Dictionary.com Unabridged, Dictionary.com, LLC, 1995–present.
• Oxford English Dictionary, 2nd ed., 1989.