Doubtless means almost certainly. Doubtless he will come, doubtless true, doubtless a mistake.
It is weaker than it looks: doubtless usually signals a confident guess rather than certainty, and it often carries a shade of irony — doubtless he had his reasons. Undoubtedly is the stronger word when you mean it. Note the silent b, added to show the Latin, as in debt.
1adjectivedoubtlessAlmost certainly.
“You have doubtless heard the news by now, so I won't repeat it.”
doubtless is a marker of confidence.
Middle English douteles is doubt plus -less. The silent b in doubt was added later to show the Latin dubitare.
Middle English
douteles
English, today
doubtless
From Middle English douteles, dowteles, equivalent to doubt + less.
Etymology from Wiktionary, CC BY-SA