Thong of Shielding

"Wait, just how is that supposed to protect her from back attacks, anyway?"

Nipple-and-Dimed for the backside. Provided that the gluteal cleavage (that's the bit between the buttocks) is covered, you can show a massive amount of rear and get away with it.

So, you could say that it's the bare minimum?... we'll get our coats...

Often a feature of the Barely-There Swimwear. Compare Chainmail Bikini, Underwear of Power.

Examples of Thong of Shielding include:

Anime and Manga

Naga from the Slayers prequels is never told to put some pants on, even though she runs around in a purple leather thong.
Shirow Masamune seems to have a deep love of this trope. The Major in the original Ghost in the Shell manga frequently shows her rear off. Although she's far more modestly dressed most of the time than in any of the adaptations. Motoko Aramaki in the sequel manga Man-Machine Interface, on the other hand, seems to be lacking pants more often than not.
Shirow Masamune's other heroine, Deunan Knute from Appleseed, whenever she is piloting a landmate in that manga.
Two of the over-the-top super-fanservice outfits in Daphne in the Brilliant Blue (Rena's and Shizuka's, to be specific) are either C-Strings (just picture a snap-bracelet for your crotch) or Maebari. Maebari deserves special mention, as it's a Japanese "clothing" fetish that revolves around bikini bottoms made out of tape. So pick your poison—strapless thong made out of a snap-bracelet, or strapless thong made out of tape.
Mirajane from Fairy Tail has this in her demon form.

Comic Books

Psylocke from the X-Men. Sometimes her costume covers her behind completely and just shows off her thighs. Other times, it could generously be called a g-string. It doesn't help that almost every panel she's in has her striking a pose like something out of a rap video.
Some modern comic book artists depict Wonder Woman's shorts (often referred to as "star-spangled panties" by fans) as this, most notably Ed Benes and Mike Deodato (who drew Wonder Woman between 1994-1995).
Green Space Babe Gamora's costume is depicted this way in the early issues of Nova v. 4 (and to a lesser extent in Guardians of the Galaxy).
Eva Procorpio in Shakara rarely has anything more than a miniscule thong between her belly and mid-thigh.
Ms. Marvel's leotard is sometimes depicted as a thong.
Despite being a robot (and functionally naked), Arcee from Transformers has occasionally been depicted with one of these.

Film

In Ralph Bakshi's Fire and Ice, all of the men's Loin Cloths bare a fair amount of buttock, but the most guilty (unsurprisingly) is Princess Teegra. Not only is her thong very narrow, but it's partly see-through, and a bit of buttcrack sticks out over the top. Plus she spends a lot of time with said ass pointed at the camera. And since this film came before the PG-13 rating, it's only PG.
In the 1999 A Midsummer Night's Dream, which featured topless fairies and a mud-slinging Hermia and Helena, features, when the nude lovers are awoken in a field by Theseus, a strategically placed blade of grass that hovers perfectly between the camera and Christian Bale's bare derriere. That could not have been accidental.
Anck-Su-Namun in the opening flashback of The Mummy 1999 sports this as we get a rear shot of her walking into Imhotep's chambers. PG-13 movies seem to be able to get away with this, though the shot can't really linger.
Transformers: Revenge of the Fallen (featuring Megan Fox) shows one of the actors in a thong. It's John Turturro.

Literature

In the 1980's action series SOBS, two members of the Heroes-R-Us unit strip off to follow the villains into the water. When the chase ends up back on land, one of them realises his colleague is effectively "bottomless" as he's just wearing a thong. When the embarrassed colleague asks for help covering up, he replies, "And deprive all these tourists of a chance to see the Eighth and Ninth Wonders of the World?"

Live-Action TV

Two examples from Britain's Got Talent. The first was "The Deans of Magic", described by Simon Cowell as "a bit Readers' Wives'' (that's a UK porn mag).
Three or four recurring competitors on Ninja Warrior. All male.
On the August 26, 1999 episode of WWF SmackDown, Tori (Terri Poch) was scheduled to face Ivory in an Evening Gown Match. Michael Cole, still a neutral character, attempted to interview Tori backstage before the match. Tori pulled down her evening gown, saying that there was no way Ivory was going to humiliate her, and, with her back to the camera, walked around backstage wearing only a thong and shoes until Road Agent Tony Garea gave her his shirt to wear. Tori won, of course, in 57 seconds.
Pants Off Dance Off would censor out the backside of any nude dancers; but not if they had a thong, and sometimes the dancers could get away with showing even more than that uncensored by intentionally covering only the smallest possible amount of skin.

Video Games

Ayla in Chrono Trigger. How her skimpy Catgirl outfit provides any protection against, say, a magic lightning bolt or a machine gun is the same principle that lets her beat giant dinosaurs and robots to death with her fists.
And on a similar note, Sira from Albion. 100% catgirl-mage who can't even be equipped with decent armor. If it weren't for her serious Magikarp Power and magical items, she'd be little more than furry eyecandy. At least the rest of the party can shield her (and other mages) from direct physical attacks, unlike in many other RPGs.
Promo material for StarCraft: Ghost.
Then there's Ashley Riot's infamous "assless chaps" in Vagrant Story—basically a huge set of trunks (or even bermuda shorts) that are missing a vital square of fabric precisely over the gluteal area.
A similar (lack of) costume is worn by the heroine of Oniblade (a.k.a. X-Blades).
Prince of Persia: Warrior Within has one worn by Shahdee in combination with a serious case of Chainmail Bikini. Shown most prominently in her introduction with a six-second close-up of her ass while she simply walks up a set of stairs. Parodied, in this strip of Penny Arcade.
Lara's ridiculous legless wetsuit in Tomb Raider: Underworld. Which is more of a leotard than anything.
Nearly every single Viera in Final Fantasy XII, but Fran is the most noticeable due to more screentime. Some of the Viera in the village were even less covered.
Ivy's costumes seem to show more and more of her buttocks with every new game in the Soul Series. And her boobs. Don't be surprised if by Soul Calibur V her costume is simply drawn on with a purple Sharpie.
- Oddly enough, her default costume in SCV is surprisingly modest, with less skin showing and not much more than a Cleavage Window.
Wario Man in Super Smash Bros. Brawl. Fortunately, his cape covers up his rear most of the time.
Cammy from the Street Fighter series. In Street Fighter IV, every battle she's in begins with an ass-shot. In Street Fighter II, roughly half of the battles she wins end in one.
Felicia from the Darkstalkers series fights other denizens of the dark in barely anything at all. Her Fur Bikini doesn't cover much and it barely even qualifies as a G-string, let alone an outfit.
The Papillon Suit in P.N.03 features a thong and assless chaps. Also a subversion as it provides exactly as much protection as one would expect it to, turning Vanessa into a One-Hit-Point Wonder.
Early in World of Warcraft they played this trope depressingly straight—one of the plate/paladin armor types looked like a thong and a bikini top made out of seashells, for example. (GU Comics even had a comic about it.) Partially due to the fan outcry, modern day generic armor is better, but the trope is not completely averted. However, the endgame "sets" that people get at level 70 and 80 do completely avert this trope—they look the exact same regardless of gender.
- The Shivarra are large, six-armed demon women with bared midriffs, and a g-string covering the rear. It's no wonder that Warlocks who upgrade their summons get one to replace their Succubus.
Promotional art for Dragon Quest IX shows one lady in this, and brandishing a sword.
Tales of Symphonia: Sheena's unlockable bathing suit costume is dangerously close to this.
Female Malkavians in Vampire: The Masquerade Bloodlines. Their range of armor seems to be stolen from a porn set. The "heavy clothing" armor is a cowboy hat, vest, buttless chaps and a thong. And combat scenes are shown in third person from behind, so...
Buki's armor upgrades in Sudeki get progressively skimpier. In the end, she's wearing a thong leotard made from Scales of steel.
Mortal Kombat:
- Kitana, Jade, Mileena, and Skarlet all wear a similar thong outfit in Mortal Kombat 9. An odd clothing choice, considering these ladies often fight fully-armored opponents who have no qualms about breaking bones and crushing body parts. Sindel's non-fatality victory pose is a Sexy Walk where she turns her back to the viewer (the thong costume showing much of her cheeks), and makes a suggestive gesture with her hips before doing a levitation spin.
- Serena has an interesting variation in Mortal Kombat: Armageddon, as the thong is covered by a skirt with crossing leather straps forming the seat, sort of a thong version of a Cleavage Window.
Played straight or averted, depending on the armor in question, in Vindictus. There are a few female-only outfits that seem to focus on sexuality, but these are nearly always cloth armor, which are meant for mages and not really focused around defense power in the first place. In general, the actual heavy and plate armors cover you completely and look the same on either gender.
The trailer for the Android system's War 1937 has a character who manages to do this with Powered Armor. (Rather shamelessly.)

Western Animation

Brenda from Perfect Hair Forever, whose Sailor Fuku skirt begins and ends up at her waist, displaying her tiny thong at all times.
Aeon Flux dresses like this, and it's strongly hinted to be a popular style among Monicans. Believe it or not, a woman attending 2011 Comic Con was asked to leave by police because her Aeon Flux costume showed too much of her butt!
Lampshaded in the Harley Quinn episode "Batman Begins Forever". Harley finds herself in a Battle in the Center of the Mind, the mind in question being Batman's, and in one part, she finds herself in Robin's role, wearing a Sexy Whatever Outfit version of the Boy Wonder's costume with this feature. Despite her usual costume having booty-shorts, she finds this a little excessive.