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Webster 1913 Edition

Adj.

Indubitable;

as, an

undubitable

principle

[Obs.]

Locke.

Adj.

Not to be doubted; unquestionable. [But the word now used is indubitable.]

undubitable ‎(comparative more undubitable, superlative most undubitable)

(rare) Alternative spelling of indubitable
- 1661, Thomas Salusbury (translator), Galileo's Dialogue Concerning the Two Chief World Systems,
  If therefore we were assured that the Earth is illuminated by the Sun, like one of those Clouds, it would be undubitable, but that it would be no less shining than the Moon.