In the Japanese anime fandom, haitenai はいてない, literally "to not be wearing," refers to a situation in which panties should be visible, and yet you aren't seeing them, making you question whether a character is wearing panties or not.
This mysterious phenomena occurring in anime has also been called in English by the names of "Schrodinger's panties" or "quantum panties," due to the indeterminacy of its plausible existence and lack of counter-evidence.
It's also called pantsu haitenai ぱんつはいてない, "not wearing panties."
And I'm writing this article, of course, because a KonoSuba sequel was announced. Aqua, one of the KonoSuba characters, wears one such Schrodinger's panties—or... doesn't—that will make you say: wait a second... she's not wearing any, haitenai!
As for what haitenai normally means in Japanese, it's the negative te-iru form of the verb haku 履く, which means "to wear," but only applies to garments which pass through your legs, such as pants, skirts, underwear, socks, and shoes.
- haku 履く
To wear. - haiteiru 履いている
haiteru 履いてる
To be wearing. - haiteinai 履いていない
haitenai 履いてない
To not be wearing.
The reason why haitenai became a thing in first place was because of fanservice. But not because people liked fanservice, because people started to despise fanservice.
As panty shots, pachira パンチラ, and other forms of chiralism became more and more popular in manga and anime, authors started to get more and more lazy about how they did it.
Girls tripped and fell onto the ground all the time with their legs open. The slightest breeze of wind turned any female character into Marilyn Monroe. One particularly egregious example is that the protagonist of Negima causes wardrobe malfunctions to every girl in line of sight every time he randomly sneezes.
It got to the point where even in angles that should be physically impossible to see panties, the skirt would get possessed by some poltergeist and flip around just to show the panties. It felt like the authors were practically saying: "you want to see some panties, right? Here. Here are your damn panties, you filthy degenerate." And I'm not talking about IyaPan.
In a tiny microcosm of the world, this cheap panty-flashing culture finally annoyed enough people to spawn a counter-culture, which is haitenai. In contrast to showing the panties all the time no matter what means necessary, haitenai is about never showing the panties, no matter how silly it looks.
But just because you can't see the panties that doesn't mean they aren't there. They're there. They're probably there. I think. Possibly. I mean, we don't have conclusive evidence of the contrary. The panties are aliens. They are there. Somewhere. We just can't see them. No matter how hard we search for them.
If a character is provably not wearing panties, by a canonical witness account, like someone stole their panties or something, so you know for sure they aren't wearing any at a given time, then it's ironically not called haitenai anymore in the fandom, even though the word literally means "not wearing it."
Instead, "no panties" is noopan ノーパン. Similarly "no bra" is noobura ノーブラ.
