Nothing but Skin and Bones

A character is shown as exaggeratedly skinny, usually for any of the following reasons:

A character hasn't eaten anything in a long time (or a short time) and it's just a joke to exaggerate how long they've been starving or how hungry they are.
A character is about to be eaten by a monster or predatory animal and they make themselves look this way so the creature won't want to eat them.
Supermodels and people stranded on Deserted Islands are often depicted this way.

Most often Played for Laughs, this is usually just a short Sight Gag with no relevance on the plot. Plot-relevant examples are much less common, as they're rarely funny.

Sometimes crosses with Lean and Mean.

Examples of incredibly thin characters with no visible bones instead fall under Noodle People.

Examples of Nothing but Skin and Bones include:

Examples where this is used as a short sight gag:

Bugs Bunny and Daffy Duck have been known to show skinnier frames under their skins to try to deter those who wish to eat them.
In the My Little Pony: Friendship Is Magic episode "Hearts and Hooves Day" the Cutie Mark Crusaders have an Imagine Spot where a famine strikes Ponyville and when one pony momentarily stands up right in front of the "camera" she suddenly has protruding ribs and a concave stomach.
In an episode of Ren and Stimpy, the duo are starving and Ren opens his skin to show there's literally no fat, just bone. Then Stimpy opens his skin to show he's nothing but skin and fat.

Nagato, a villain of Naruto, is horribly skinny because of a Dangerous Forbidden Technique of his, which drained his energy. His legs were burned useless just before that. And overall, he uses nothing but techniques that consume huge amounts of Chakra. Here is the result.
Yagi Toshinori All Might's true form from My Hero Academia is wasted and gaunt from his injuries, and he frequently coughs up Blood From the Mouth. These health issues are what cause him to become a Retired Badass.

Astral Journey: It's Complicated: Melanie is yelled at by Mel, who tells her that she can see her rib-cage. Melanie had a combination of an eating disorder and worms making a meal of her. In fact, Melanie had to have surgery to remove them. As a result, she confined to a wheelchair and/or stretcher as a result.

In The Quest for Saint Camber, Nigel is described as wasted and frail some two weeks after Conall attacked him with magic and left him in a coma. Since Nigel got no solid food for that period, it's entirely plot justified and not pretty. Morgan and Duncan leave Rhemuth to search for Kelson and Dhugal partly to avoid watching Nigel starve to death.

Seen on Intervention either because the person has been consuming mostly drugs and little food, or has an eating disorder.
Subverted in an episode of The Drew Carey Show. Drew is on a hunger strike and when Mr. Wick comes to see him at home, Wick finds Drew lying on the couch due to lack of energy and is now extremely skinny. But it turns out that Drew knew Wick was coming over and he got an assist from a pizza delivery man who is naturally extremely skinny. The pizza guy is willing to help because Drew on a hunger strike is costing him business. They lay on the couch together, covered by a blanket, arranged in a way that it looks like Drew's head is on the other guy's body.

And the kick is so divine when she sees bones beneath her skin

A short sequence of some early Dilbert strips observed Dilbert's date with a supermodel, who was drawn as a literal skeleton, and did not eat on their dinner date but instead simply sniffed the mints.
In Peanuts, Snoopy's desert-dwelling brother Spike was like this initially. In the first strip where he appeared, he woke up after traveling a long time, and Snoopy announced, "Eggs benedict for my brother Spike!" To which Lucy replied, "Better make that ten pounds of buffalo steak" before he was actually seen. In later stories, Spike was simply thin.

In the Forgotten Realms setting, Chosen Ones are humans transformed into humanoid monsters by the Red Wizards of Thay. They are downright morbidly thin.

In World of Warcraft, Nightfallen are night elves from Suramar City who are separated too long from the Nightwell, a source of power that sustains them. The gradual starvation turns them into gaunt, frail creatures. It is possible to stabilize them in this state if their hunger is sated, or cure them completely if a connection to the Nightwell is reestablished (the player's goal). If neither is done, starvation eventually causes them to become Withered, a Fate Worse Than Death. Withered are even more gaunt, and have become mindless, feral monstrosities who attack anyone or anything with Mana.

Issue 8 of Flying Suit Reiko has Reiko getting her overweight friend Potchari to take a fitness test for her so she can take diet pills she's inelligible for and throughout the story she gets skinnier and skinnier.
Sinjal/Crippled from Wurr is very, very skinny. While not underfed per se (not anymore than, say, Morri) he has severe disgestion problems.

In one Quick Draw McGraw cartoon, a wily mountain lion steals one of Quick-Draw's biggest, fattest sheep, then shears it down to prepare it for dinner. It is then that he discovers that the sheep is rail thin underneath its wool.
In Adventure Time, The Ice King is actually very skinny under his robe.