Definition: Doubtless, without doubt, unquestioned; unquestionable.
[@more@]Usage: Both "doubtless" and "indubitable" may be used as adverbs but you must add -ly to the latter: "Indubitably/Doubtless he left early." Both may be used in the predicate: "His wisdom is indubitable/doubtless" Only "indubitable" may be used attributively: "His indubitable wisdom failed him." "Doubtless" does not work here.
Suggested Usage: Use this adjective where you would want to say "undoubtable" (?): There has been indubitable failure of communication here," or "I think it indubitable that you misspoke yourself just now." (This word was suggested by YDC friend Dave Rosenzweig.)
Etymology: From Latin indubitabilis "doubtless, not wavering in opinion" from in- "not" + dubitare "to be uncertain" + abil "able-to." The PIE root dwo- developed into Latin "duo" and English "two". The original meaning was "wavering back and forth." "Doubt" derives from a suffixed version of the same root.
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