Definition of "incog"

Though we travel'd incog. yet we trembled with fear,For the accents of strangers fell hoarse on our ear.

1846, Lydia Huntley Sigourney, "Forgotten Flowers", Voice of Flowers, page 64.

But his general aspect and manner were so suggestive of an education and career incongruous with his naval function that when not actively engaged in it he looked like a man of high quality, social and moral, who for reasons of his own was keeping incog.

1985, Herman Melville, Billy Budd, Sailor, and Other Stories, Penguin Classics, Harmondsworth, page 343

What—my old Guardian—what[!] turn inquisitor and take evidence incog.—

1777, Richard Brinsley Sheridan, The School for Scandal, IV.iii

Then you, ye auld, snick-drawing dog!Ye cam to Paradise incog,An’ play’d on man a cursed brogue,(Black be your fa’!)

1786, Robert Burns, Address to the Deil

“Just as we arose from the table some county judges came in and the incog was off.”

2009 April 5, Matthew Algeo, “Harry Truman, Leader of the Freeway”, in New York Times‎