A hair shirt is a garment or undergarment made of coarse animal hair, worn close to the skin. It is designed to irritate the skin, and worn as a self-imposed means of repentance and mortification of the flesh.
In Essos, the followers of Boash, the Blind God, ate no flesh and drank no wine. They went about barefoot, clad only in hair shirts and hides. Their priests were eunuchs, who wore eyeless hoods in honor of their god. 1322 years before the Doom of Valyria, a sect of this religion left the Valyrian Freehold to establish a temple and colony in Lorath. Over time, the worship of Boash dwindled, and the priests grew worldly and corrupt, forsaking their hair shirts, hoods, and piety, and growing fat and rich off the taxes they extracted from those they ruled.
Norvos was another Valyrian colony founded by religious dissidents. The bearded priests are a sect so secretive that even the name of their god is revealed only to initiates. He is a stern god, as the priests wear hair shirts and untanned hides, and practice ritual flagellation as part of their worship. Once initiated, they are forbidden to shave or cut their hair.
In Westeros, the Warrior's Sons were an order of knights sworn to the Faith of the Seven, part of the Faith Militant. They wore rainbow cloaks, and inlaid silver armor over hair shirts. The Warrior's Sons were outlawed during the Faith Militant uprising by King Maegor I Targaryen, and officially disbanded in 48 AC during the reign of his successor, King Jaehaerys I Targaryen.
In 130 AC, during the Dance of the Dragons, a begging brother known as the Shepherd arose in King's Landing, a barefoot scarecrow of a man in a hair shirt and roughspun breeches, filthy and unwashed and smelly. The Shepherd preached against Queen Rhaenyra Targaryen and dragons, inspiring the storming of the Dragonpit during the riots in King's Landing. He led a "barefoot army" until he was captured and executed by King Aegon II Targaryen.
A Clash of Kings
At Dragonstone, Salladhor Saan wonders if King Stannis Baratheon would like him better if he wore a hair shirt and never smiled, but rejects the idea and tells Davos Seaworth that the king must tolerate him in silk and samite.
A Feast for Crows
In King's Landing, Queen Regent Cersei Lannister tells her companion, Taena Merryweather, about the practices of the Warrior's Sons and the hair shirts they wore under their armor. Cersei arranges with the High Sparrow to overturn Maegor the Cruel's law which forbade holy men from being armed, and the orders of the Faith Militant are reborn.
At Darry, Ser Jaime Lannister encounters his cousin Lancel Lannister praying in the sept, thin and barefoot, looking like a beggar in his roughspun tunic and hair shirt. When Jaime asks Lancel what he did to deserve such atonement, Lancel confesses that he helped Cersei murder her husband, King Robert Baratheon, and also slept with her. Lancel further explains that he is renouncing his titles, land, and bride, and joining the High Sparrow and the Warrior's Sons.
A Dance with Dragons
As Cersei enters the Hall of Lamps of the Great Sept of Baelor before she is to begin her walk of atonement, she finds a dozen of the new Warrior's Sons waiting to accompany her. Cersei sees their rainbow cloaks, their crystal-crested greathelms, and their silver plated armor, but she knows that underneath, each knight is wearing a hair shirt.
- Wikipedia: Hair shirt
- The World of Ice & Fire, The Free Cities: Lorath.
- The World of Ice & Fire, The Free Cities: Norvos.
- A Feast for Crows, Chapter 28, Cersei VI.
- A Feast for Crows, Chapter 33, Jaime V.
- Fire & Blood, Prince into King - The Ascension of Jaehaerys I.
- Fire & Blood, The Dying of the Dragons - Rhaenyra Overthrown.
- Fire & Blood, The Dying of the Dragons - The Short, Sad Reign of Aegon II.
- Fire & Blood, Aftermath - The Hour of the Wolf.
- A Clash of Kings, Chapter 10, Davos I.
- A Feast for Crows, Chapter 30, Jaime IV.
- A Dance with Dragons, Chapter 65, Cersei II.