DOUBTLESSLY
undoubtedly, doubtless, doubtlessly
(adverb) without doubt; certainly; “it’s undoubtedly very beautiful”
Source: WordNet® 3.1
Etymology
Adverb
doubtlessly (comparative more doubtlessly, superlative most doubtlessly)
(sometimes, proscribed) Without question; indubitably.
Usage notes
Sometimes considered a redundancy, as doubtless (without the -ly suffix) functions as an adverb with an identical meaning. The synonyms indubitably, undoubtedly, and unquestionably all require the -ly suffix.
Source: Wiktionary
Doubt"less*ly, adv.
Definition: Unquestionably. Beau. & Fl.
DOUBTLESS
Doubt"less, a.
Definition: Free from fear or suspicion. [Obs.]
Pretty child, sleep doubtless and secure. Shak.
Doubt"less, adv.
Definition: Undoubtedly; without doubt.
Source: Webster’s Unabridged Dictionary 1913 Edition