Without doubt; certainly, unquestionably, or with certainty.
Formed from 'doubtless' (adjective, from 'doubt' plus '-less,' meaning 'without') plus the adverbial '-ly.' The word has been used since the 1500s to express certainty, ironically being the opposite of 'doubtfully.'
Doubtlessly is one of English's most interesting contradictions—we use a word literally meaning 'without doubt' (doubtless + -ly) to emphatically agree with something, even though the '-less' ending already makes it an adjective, so 'doubtlessly' is technically redundant!
Complete word intelligence in one call. Free tier — 50 lookups/day.