When was the fantasy trope of psychological invisibility first used?

Question

In the Enchanters’ End Game, the last book of the Belgariad series, the following exchange between Belgarath the sorcerer and Silk takes place, when they discuss how the Sword has been hidden:

“It’s still there,” Silk noted, sounding a bit disappointed. “I can still see the sword.”
“That’s because you know it’s there,” Belgarath told him. “Other people will overlook it.”
“How can you overlook something that big?”, Silk objected.
“It’s very complicated,” Belgarath replied. “The Orb is simply going to encourage people not to see it - or the sword. If they look very closely, they might realise Garion is carrying something on his back, but they won’t be curious enough to find out what it is”.

To me, this sounds a lot like the description of Somebody Else’s Problem:

The Somebody Else's Problem field... relies on people's natural predisposition not to see anything they don't want to, weren't expecting, or can't explain. If Effrafax had painted the mountain pink and erected a cheap and simple Somebody Else’s Problem field on it, then people would have walked past the mountain, round it, even over it, and simply never have noticed that the thing was there.

The German role-playing game The Dark Eye also has a similar spell, called Harmless Shape:

You take the shape of an inconspicuous person (such as a servant or beggar) that does not attract attention at the current location. The illusion changes your appearance and voice, but does not grant knowledge of things like languages or appropriate behavior. This spell does not hide larger objects or familiars. The spell itself picks the shape automatically—the caster cannot choose which appearance to take.

As the comments have pointed out, Terry Pratchett used a similar idea, and TVTropes lists something along the lines as well.

I was wondering, when and where was this idea first mentioned?

Answer 1

@eike seems to be correct, that the first mention of psychological invisibility is in G.K. Chesterton's short story, "The Invisible Man", published in 1911.

It is predicated on the notion that the people assigned to watch for the murderer actually saw him, but ignored and didn't remember him because he was so commonplace and unremarkable.

When those four quite honest men said that no man had gone into the Mansions, they did not really mean that no man had gone into them. They meant no man whom they could suspect of being your man. A man did go into the house, and did come out of it, but they never noticed him."

"An invisible man?" inquired Angus, raising his red eyebrows. "A mentally invisible man," said Father Brown.

It also has science fiction elements, namely "clockwork" servants/automated dolls:

"I use them in my own flat," said the little black-bearded man, laughing, "partly for advertisements, and partly for real convenience. Honestly, and all above board, those big clockwork dolls of mine do bring your coals or claret or a timetable quicker than any live servants I've ever known, if you know which knob to press. But I'll never deny, between ourselves, that such servants have their disadvantages, too."

The next chronologically that I can find is the 1930's radio series "The Shadow", who has the power to cloud men's minds so they don't see him. This debuted in 1931, and the radio show in 1937:

On September 26, 1937, The Shadow radio drama, a new radio series based on the character as created by Gibson for the pulp magazine, premiered with the story "The Death House Rescue," in which The Shadow was characterized as having "the power to cloud men's minds so they cannot see him." As in the magazine stories, The Shadow was not given the literal ability to become invisible.

Originally attributed to hypnotism, it was changed to psychic powers during the rewrite of the stories in 1963:

In these novels, The Shadow is given psychic powers, including the radio character's ability "to cloud men's minds", so that he effectively became invisible; he is more of a spymaster than crime fighter in these updated eight novels.

One outside possibility, is "The Horla", by Guy du Maupassant, first published in 1885. In it, the creature is not invisible, per se, but it is that the human eye is not capable of seeing it. I doubt this really qualifies, since the person involved eventually knows that the creature is there, but can't see it.

Answer 2

If you're looking for something magical rather then purely psychological, an early usage was by Randall Garret in Too Many Magicians, Analog, 1966 as the "Tarnhelm Effect":

"The Tarnhelm Effect?" asked Master Sean. He chuckled. "My lord, regardless of what the layman may think, the Tarnhelm Effect is extremely difficult to use in practice. Besides, 'invisibility' is a layman's term.

Spells using the Tarnhelm Effect are very similar in structure to the aversion spell you met at the door to this room. If a sorcerer were to cast such a spell about himself, your eyes would avoid looking directly at him. You wouldn't realize it yourself, but you would simply keep your eyes averted from him at all times.

He could stand in the middle of a crowd and no one could later swear that he was there because no one would have seen him except out of the corner of the eye, if you follow me.

"Even if he were alone, you wouldn't see him because you'd never look at him. You would subconsciously assume that whatever it was you were seeing out of the corner of your eye was a cabinet or a hatrack or an umbrella stand or a lamppost—whatever was most likely under the circumstances.

Your mind would explain him away as something that ought to be there, as a part of the normal background and therefore unnoticeable.

"But he wouldn't actually be invisible. You could see him, for instance, in a mirror or other reflecting surface simply because the spell wouldn't keep your eyes away from the mirror."