The Collaborative International Dictionary of English v.0.48:
Doubtlessly \Doubt"less*ly\, adv. Unquestionably. --Beau. & Fl. [1913 Webster]WordNet (r) 3.0 (2006):
doubtlessly adv 1: without doubt; certainly; "it's undoubtedly very beautiful" [syn: undoubtedly, doubtless, doubtlessly]