Cannibalism

Cannibalism is the act or practice of humans eating the flesh or internal organs of other human beings.

In Westeros, there is one known society that practices cannibalism - the Thenn people, who feast on the flesh of their enemies.

Season 1
Ser Alliser Thorne tells Jon and Sam that during the last winter, Thorne and his ranger companions were trapped in a massive winter storm and ran out of food; first they ate the dead horses, and later had to cannibalize the frozen remains of the dead comrades. Thorne nastily comments that Sam would have fed them for two weeks, and they'd have had his bones left over for soup.

Season 2
Following the battle of Oxcross, the Lannisters spread false rumors that the Northmen feasted on the flesh of their dead foes.

Bronn tells Tyrion and Varys that during a siege, it is not the fighting but the starving that kills most people; if things get bad enough, the poor people start eating each other.

Season 4
Tormund and Ygritte encounter Styr, the Magnar of the Thenn, and his party. Styr tells them that his party "got some supper" from a village. As his men roast a human hand over a fire, Styr fantasizes aloud about eating crows.

In the books
In the A Song of Ice and Fire novels, there are two known societies which are rumored to practice cannibalism: the inhabitants of Skagos and the ice-river clans of the Free Folk. According to The World of Ice and Fire, there are additional societies that allegedly practice cannibalism: the Adakhakileki, the inhabitants of the Cannibal Sands, and the Brindled Men who inhabit the south regions of Sothoryos.

It should be noted that the rumors about the aforementioned societies have not been verified.

In contrast to incest, kinslaying and violation of the guest right, there is no mentioning in the novels that cannibalism is considered as abomination by any of the Westerosi religions, or socially unacceptable. A very few book characters (Stannis and Dagon Codd) have expressed their disapproval of cannibalism, to the point of killing those who have been caught eating dead bodies.

There are horror stories about mentally ill people who used to eat human flesh:
 * Danelle Lothston used to bathe in tubs of blood and preside over feasts of human flesh at Harrenhal.
 * The Amethyst Empress's younger brother, who cast her down and slew her, was said to have practiced cannibalism.

The Mountain, at Tywin's command, is sent to Harrenhal to kill Vargo Hoat for his betrayal. Rather than simply kill Hoat, the Mountain cuts his limbs piece by piece, and feeds them as a "roast goat" to northern prisoners (among them Ser Wylis Manderly), even to Hoat himself. Jaime is filled with disgust to hear that; he wonders if the captives had any idea what they'd been eating, and decides it is better not to inquire.

There have been cases of resorting (or almost resorting) to cannibalism due to lack of food:
 * During the Siege of Storm's End, Ser Gawen Wylde and more of Stannis's garrison planned on sneaking out and surrendering to the Tyrell army. Stannis wanted to send them to the Tyrells by catapult, but Maester Cressen persuaded him to keep the would-be traitors imprisoned, just in case the garrison ran out of food; it is unclear whether Cressen actually meant it, or he said it in order to dissuade Stannis from excessive cruelty that might have turned his troops against him. Luckily for the defenders, there was no need for that, since Davos smuggled into the castle enough food to sustain them till Eddard Stark arrived and lifted the siege.
 * Euron Greyjoy tells his brother Victarion that he captured once four warlocks; one of them threatened him, so Euron killed and fed him to the other three. They refused to eat at first, but when they grew hungry enough, they had a change of heart. Since Euron is highly untrustworthy, it is unknown whether the story is true; it fits, however, Euron's cruel nature and his habit of tormenting and playing vicious mind games on his captives.
 * When Theon arrives at Moat Cailin, he is told by one of the ironborn guards that Dagon Codd caught two ironborn eating dead bodies, and killed them both.
 * While the Yunkai troops besiege Astapor, the besieged soon run out of food; first they eat cats and rats and leather, and finally resort to cannibalism: they gather in secret to draw lots and gorge upon the flesh of the losers.
 * During the march of Stannis's host to Winterfell, four of Lord Peasebury's men are caught butchering and roasting one of the late Lord Fell's men. Asha Greyjoy (Yara's name in the novels) is horrified to hear that; she assumes those four were not the first to taste human flesh during the grim march - only the first to be discovered. Stannis sentences the four to death by burning. Much to Lord Peasebury's embarrassment, there are rumors that he knew what his men were doing, and might even have shared in their feasts. When he calls Ser Justin Massey craven, the latter retorts "Better a craven than a cannibal".

In real-life
Cannibalism has been well documented around the world, from Fiji to the Amazon Basin to the Congo to the Maori people of New Zealand, especially among tribal societies, as a cultural norm. There is evidence that cannibalism has been practiced for hundreds of thousands of years, from the early stages of mankind to the modern era, initially due to food shortages, and over the years it has become a social custom.

The reasons for cannibalism among tribal societies have been the belief that by consuming a dead body - the eater gains the deceased's valued qualities, and also a symbolic expression of the domination of an enemy in warfare.

Cannibalism has been occasionally practiced as a last resort by people suffering from famine, even in modern times. For instance, multiple acts of cannibalism were reported during the Soviet famine of 1932–1933.