doubtless (adv., adj.), doubtlessly, indubitably, no doubt, undoubtedly, unquestionably, without doubt (advs.)
Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
Doubtless can be an adjective meaning “free of doubt,” but it is relatively rare: He is doubtless and fearless when he begins his performance. Doubtless is also an adverb, as are all the other locutions in this entry; all mean “probably,” “certainly,” or “absolutely.” Doubtless and doubtlessly (which is rare and often considered clumsy since the flat adverb doubtless is available too) may not always sound very certain; they usually express the sense of “probably, very likely.” No doubt is perhaps a bit stronger, but not quite certain in some contexts; in others it shares the force of undoubtedly. Undoubtedly and without doubt are stronger; they express certainty. Indubitably and unquestionably are the most forceful of all. They express dead certainty, leaving not the shadow of a doubt. Try each of them in place of doubtless in this sentence for an idea of the gradations: It will doubtless rain this afternoon.
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