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The Collaborative International Dictionary
Undoubtable

Undoubtable \Un*doubt"a*ble\, a. Indubitable.

Douglas Harper's Etymology Dictionary
undoubtable

early 15c., from un- (1) "not" + doubt (v.) + -able. Related: Undoubtably.

Wiktionary
undoubtable

a. (context rare English) Incapable of being doubted; indubitable.