Fundoshi

The fundoshi (Japanese: 褌 / ふんどし) is the traditional Japanese style of men's Loin Cloth, although it's also worn by women. It is a single long piece of cloth wrapped into place and fastened by twisting and tucking alone. This versatile tying technique makes any suitable piece of cloth a potential fundoshi. A similar technique can also be used to wear cloth as a headband.

Traditionally, the fundoshi was worn as a form of underwear. Its role as underwear has decreased in modern Japanese society, as more people wear Western-style boxers, briefs, panties, etc. The fundoshi still may be worn during traditional festivals, or as swimwear.

In the modern world, the fundoshi has been noticed for another quality. The fact that it covers the genitals while leaving almost everything else exposed, has made it a well-acknowledged Fetish Fuel. It has been adopted as a form of Fan Service, with a similar effect as Barely-There Swimwear.

In Japan, this is also less of an issue for the Moral Guardians, as the fundoshi is a well-recognized part of Japanese traditional culture. Thus it is actually in relatively better taste than a speedo, despite actually exposing more skin than a speedo. Outside of Japan, the fundoshi has become increasingly popular as a form of sexy underwear or fetish wear for men.

Here's how to tie a fundoshi. (Practical, demonstrative, and not explicit but for obvious reasons may be Not Safe for Work.)

A Sub-Trope of Loin Cloth.

Compare Barely-There Swimwear, Sweater Girl, Wet Sari Scene (both of which are also Distaff Counterparts).

Examples of Fundoshi include:

Anime and Manga

In Kenichi: The Mightiest Disciple, Shigure and Hayato prefer fundoshi to Western-style bathing suits. Shigure also wears a chest wrap for her upper body and finds a standard two-piece bathing suit to be indecent.
- Shigure always wears a fundoshi.
In the Gokusen manga, Ooshima more or less forces Shin to participate in a festival where the men traditionally wear fundoshi, because it's major Fetish Fuel for Kumiko.
Yotsuba&!: When Yotsuba sees someone in a fundoshi at the town festival, she reacts as a five-year-old probably would: "BUTT!! HAHAHAHAHAHAHA!! BUTT!!!"
In the Ouran High School Host Club manga, Tamaki must wear one of these if his team loses the sports festival. They do. He does
Featured often in Gintama for comedic effect.
Pesche from Bleach
Another female example: Matabei Gotou from Hyakka Ryouran Samurai Girls wears this. In her case, it's definitely Fan Service... (She's not even wearing a skirt or anything over it!).
Junko's from Ichiban Ushiro no Dai Maou is first thing we see of her.
Clain from Fractale, after being forced to change his dirty pants, reveals that it's his choice of underwear.
Played for squicky laughs in a One Piece filler set in Medieval Japan, where Nami is hit by a strange piece of cloth... revealed to be Franky's Fundoshi. Her reaction is priceless.
In Sakigake Otokojuku, fundoshi are the only underwear permitted by school rules. Consequently, all the students wear them.
Baki the Grappler's Hanayama Kaoru is a Yakuza, and so a very traditional man. When he gets serious on a fight, he rips his clothes, shows his family's back tattoo and keeps on fighting. Wearing only a fundoshi.
Goemon from Lupin III. Particularly of note on missions involving swimming or diving; the rest of the Lupin gang will opt for wetsuits, but not the highly traditional Goemon.
Salamander from Yondemasuyo, Azazel-san wears nothing but a fundoshi. It's a Shout-Out to Yukio Mishima (of whom Salamander is a parody of) posing in fundoshi with a sword for a photoshoot.
In Fairy Tail, Elfman wears one adorned with a character meaning "honorable man".
Kukuri of Hell Girl sports a pair of these. Though considering she looks to be about five years old at best (Hard to tell with demons and all) it's definitely not for fanservice.
During the Beach Episode of Rental Magica, Nekoyashiki wears a fundoshi on the beach. Adelicia and Honami treat this as in-universe Fan Disservice.

Comic Books

Green Hornet: Year One. When conducting research for Kato's origin, writer Matt Wagner was surprised to discover that Japanese underwear of the period essentially consisted of - as he put it - 'banana hammocks'. A scene involving Japanese soldiers in fundoshi ended up in the comic.
In the Gargoyles spin-off Bad Guys, Yama is shown to wear a fundoshi while in stone sleep. It's also hinted from the clothing and equipment neatly laid out on the bed that he often wears an athletic cup.

Film

Ashitaka from Princess Mononoke has a fundoshi as underwear.
Many of Akira Kurosawa's movies, being set in medieval Japan, feature men wearing this (and often not much else).
In Gung Ho, the Japanese plant managers are shown swimming in a river wearing fundoshi. In a version bowdlerized for cable TV, these were digitally retouched to look more like conventional swimsuits when seen from behind.

Video Games

As a consequence of fandom injokes, Rinnosuke from Touhou (particularly his counterpart Mannosuke) is often depicted wearing nothing but a fundoshi.
Gotou in Shin Megami Tensei I sports a fundoshi despite being a military man. Hell, he will even fight you with only a fundoshi on.
- Speaking of Shin Megami Tensei, several demons such as pre-Soul Hackers Loki and Soul Hacker design Take-Mikazuchi all wear a fundoshi.
- Persona 4 has a fundoshi as armor. Unfortunately, it doesn't appear on their in-game models.
Heihachi's 1P costume from Tekken 4. Enough said....
Juuichi's fundoshi scene during the camping arc of Morenatsu. Also, Tatsuki is sometimes seen outdoors in his fundoshi.
Mizoguchi's appearance from King of Fighters: Maximum Impact 2 Regulation A, his alternate costume is nothing but a fundoshi tied around him.
The Ruan Brothers from Outlaws of the Lost Dynasty all share the same muscled physique with a rokushaku fundoshi as their base sprite. The only difference is that they put on different colored loincloth and various heads.
One of Zafira's unlockable swimsuits in Magical Girl Lyrical Nanoha A's Portable: The Gears of Destiny. The other one is an Old-Timey Bathing Suit.
Marvel vs. Capcom 2: Jin's taunt and anti-air animation show he's not afraid to be traditional in more than one way.

Western Animation

Samurai Jack wears a fundoshi under his usual outfit. Since Jack suffers Clothing Damage in half the episodes, he has many, many fundoshi scenes. This has contributed to making Jack a very potent Mr. Fanservice.

Real Life

This is considered traditional in Japan so expect to see it at almost any culture festival that involves men with special outfits. Special manliness points go to Hokkaidō men who wear only this in the snow and the Kurama hi-matsuri (fire festival), where men do this while holding 20 foot long torches on their shoulders. At the annual Kanamara Matsuri festival women wear fudoshis when they present and carry a giant pink phallus called Elizabeth. (NSFW)
Also a pride point for many Yakuza, who loudly proclaim their traditionalist ways. Anytime they pose for their elaborate back tattoos (irezumi), there is about a 99% chance that they will do it in Fundoshi.