Origin: 1300–50;Middle Englishdouteles. See doubt, -less
Synonyms 1, 2. Doubtless, undoubtedly, indubitably, unquestionably are adverbs that express certainty. Doubtless although it sometimes denotes an absolute degree of certainty, more often means “probably,” “presumably,” or “no doubt”: She will doubtless accept the offer.Undoubtedly means “beyond doubt”: undoubtedly the most prolific and popular composer of his time.Indubitably and unquestionably both affirm an unassailable conviction; they differ only in tone, indubitably being appropriate to more formal or learned discourse and unquestionably appropriate in a wider range of contexts and styles: an indubitably (or unquestionably) accurate transliteration of the hieroglyphic text; unquestionably the hottest running back in the league.