Shanks lost his left arm when he sacrificed it to save Luffy from a Sea King attack at the beginning of One Piece. Despite being one of the strongest characters with powerful Haki abilities, Shanks allowed the Sea King to bite off his arm to protect Luffy, who was vulnerable and could have drowned otherwise. This act was also symbolic, as Shanks wanted to impress upon Luffy the dangers of the pirate life and inspire him to grow strong and responsible as the future generation of pirates. Additionally, recent story developments hint that Shanks' arm might have had a special mark (the "mark of the abyss" or a symbol related to World Government or Holy Knights), and losing his arm could have also served as a way to sever ties from a darker past or lineage, adding more layers to his sacrifice.