Hiraikotsu (飛来骨, lit: "fly", "come", and "bone" respectively. Sometimes translated to mean "The bone that flies back") is the favored weapon of Sango the demon slayer from the popular anime and manga InuYasha.
The Hiraikotsu is a giant boomerang made from the bones of demons. The weapon can slice through many demons and comes in handy when InuYasha and his friends were at Mt. Hakurei. The Hiraikotsu is actually quite heavy (approximately 40 to 50 lbs) and requires a great deal of strength to throw. It can also be used defensively as a shield or a deflector of attacks.
Because the weapon is made from the bones of demons, it is usually unbreakable, though in Kaijinbo's Evil Sword, the weapon was cut in half by the possessed Kaijinbō using Tōkijin. Though powerful enough to even withstand the brunt of the Kaze no Kizu/Wind Scar (or rather a portion of it) upon being reforged at the hands of Tōtōsai, the Hiraikotsu can and has been reflected back at her by Kanna, Kagura and Princess Kaguya. Due to the weight of the weapon, it usually ends up knocking her out.
In the manga, Sango destroys the Hiraikotsu to save Miroku and herself from a bone-devouring demon by spreading a highly corrosive poison on the weapon and throwing it at the demon. As the demon dissolves from the poison, so does the Hiraikotsu and Sango ruefully apologizes for sacrificing her most trusted weapon.
In order to repair the weapon, Tōtōsai and Myōga advise Sango and her friends to visit Yakurou Dokusen, a sage of medicines and poisons. While initially reluctant, Yakurou Dokusen offers Sango a method to fix Hiraikotsu. In order to do so, she must defeat several powerful demons with her own strength and transform the poison that Yakurou Dokusen has submerged the Hiraikotsu in into medicine before the poison completely dissolves her weapon. The yōkai attacking her were revealed to be the same creatures that comprised the Hiraikotsu, who were displeased with Sango for sacrificing them for a mere human like Miroku. The yōkai offer to fight by her side once again if Sango pledges not to commit a similar betrayal in the future, a pledge she couldn't commit to, out of her feelings for Miroku. In the end, the yōkai sense Miroku's resolution, and decided to fight by her side once again as the reborn Hiraikotsu, to see the situation towards the end. This "reforging" of the Hiraikotsu ended in it being given the ability to absorb Jaki, which allowed it to cut through Naraku and even prevent the reformation of his body.
Besides being able to cut through demons, the Hiraikotsu can sometimes be combined with other weapons (usually holy). Sango uses Miroku's staff in combination with the Hiraikotsu in InuYasha the Movie 2: The Castle Beyond the Looking Glass to deal serious damage to Naraku, and uses Miroku's sutras on the weapon to increase its power on Mount Hakurei (occurs only in the anime). She also uses these attacks in the game InuYasha: The Secret of the Cursed Mask.
In an anime only episode, it is revealed that the Hiraikotsu was also used by her grandfather when he found the Shikon no Tama inside the body of a demon.