Definition of "unknown"

The Celebrity, by arts unknown, induced Mrs. Judge Short and two other ladies to call at Mohair on a certain afternoon when Mr. Cooke was trying a trotter on the track. The three returned wondering and charmed with Mrs. Cooke; they were sure she had had no hand in the furnishing of that atrocious house.

1897 December (indicated as 1898), Winston Churchill, chapter IV, in The Celebrity: An Episode, New York, N.Y.: The Macmillan Company; London: Macmillan & Co., Ltd., page 58

I suspect that this large and complex military railway system, shrouded in official secrecy for most of its operational life, remains unknown to many people.

2022 January 12, Chris Hegg, “The secret railway in the woods”, in RAIL, number 948, page 34

Had God walked close beside her into the unknown?

1957, Ethel Erford Hewitt, Into the Unknown: An Historical Novel, page 351

As we know, There are known knowns. There are things we know we know. We also know There are known unknowns. That is to say We know there are some things We do not know.

2003 , Donald Rumsfeld, edited by Hart Seely, Pieces of Intelligence: The Existential Poetry of Donald H. Rumsfeld

The other priority is getting people to respond well to interventions, especially changes to routine. This is one of the biggest unknowns in these scenarios, and yet compliance can be the most crucial factor in determining whether an intervention works.

2020 April 9, Ian Boyd, “We practised for a pandemic, but didn’t brace”, in Nature, volume 580, number 7802, page 9

How does it feelTo be on your ownWith no direction homeLike a complete unknownLike a rolling stone?

1965, Bob Dylan, Like a Rolling Stone