Definition of "doubtless"

There are those who have doubtless faith in revelation.

1880, B. Stewart, P. G. Tait, The Unseen Universe, London: Macmillan, page xiii

It is doubtless that the old reprobate who sued for his daughter's hand heard some unsavory truths from the man.

1927, Edgar Rice Burroughs, chapter 7, in The Outlaw of Torn

Pretty child, sleep doubtless and secure.

c. 1596 (date written), William Shakespeare, “The Life and Death of King Iohn”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, [Act IV, scene i]

I cannot remember that I spoke a word, though doubtless I did.

1893, Ambrose Bierce, The Realm Of The Unreal

The men, Dorothy thought, were about as old as Uncle Henry, for two of them had beards. But the little woman was doubtless much older. Her face was covered with wrinkles, her hair was nearly white, and she walked rather stiffly.

1900 May 17, L[yman] Frank Baum, The Wonderful Wizard of Oz, Chicago, Ill., New York, N.Y.: Geo[rge] M. Hill Co.

There is also in these matters of Science (though many scientific men would doubtless deny this) a great deal of "Fashion".

1920, Edward Carpenter, Pagan and Christian Creeds, New York: Harcourt, Brace and Co., published 1921, page 9

Thom Yorke’s onstage pronouncements became less abstract and more effusive, they played the songs the less committed audience members doubtless wanted to hear in the first place: Paranoid Android, Creep, Fake Plastic Trees and Karma Police. At the end of the latter, Yorke happily led an audience singalong.

2017 June 26, Alexis Petridis, “Glastonbury 2017 verdict: Radiohead, Foo Fighters, Lorde, Stormzy and more”, in the Guardian‎