what happened to shanks arm

Shanks lost his arm while saving Luffy from a Sea King during Luffys backstory. Shanks used Haki to scare away the Sea King, but still lost his arm in the process. According to a theory, Shanks bet his arm on Luffy's future, believing that the experience would motivate Luffy to become a great pirate. Shanks' missing arm has become an important part of his character, and it has been suggested that it was necessary for forming a bond between Luffy and Shanks that will have some relevance in the future. Additionally, Shanks losing his arm meant that Mihawk could no longer fight him, given that defeating a man without an arm would leave a sour taste in his mouth.