To undo the act of seeing something; to erase the memory of having seen something, or otherwise reverse the effect of having seen something.
1829, Robert Taylor, "Infidel Mission.—Fifteenth Bulletin", in The Lion, volume IV, number 10, page 304:We have shown the world, and it cannot be unseen, it cannot be unknown, it cannot be forgotten, that Christianity cannot be defended on any ground where Infidelity can get an inch of fair play against it.
1897 March 20, George Bernard Shaw, "Shakespeare in Manchester", printed in 1906, Dramatic Opinions and Essays with an Apology by Bernard Shaw, Volume 2, Brentano's (1922), page 215:I have only seen the performance once; and I would not unsee it again if I could; but none the less I am a broken man after it.
1969, Joseph McElroy, Hind's Kidnap, page 180:once you’ve seen this you bear always the burden of its sight. And, as Laura says, you can’t unsee it.
1977, Stephen King, The Shining:Once you saw the face of a god in those jumbled blacks and whites, it was everybody out of the pool—you could never unsee it.
1991, E. Roy Weintraub, Stabilizing Dynamics, page 94:Once one has “seen†the well-known gestalt psychology drawing of the young woman in a fur coat, she cannot be “unseen†after one notices the alternative, an old crone
