Heavy Metal Pirates

The Heavy Metal Pirates (ヘヴィーメタル海賊団 Hevī Metaru Kaizokudan) are a pirate crew that operates near the Kingdom of Cross, and are business partners with The Syndicate.

Etymology
Heavy metal is a genre of rock music. This would begin the theme of pirate crews being named after various rock bands. The name, however, appears to be a pun on the fact that the crew was first seen transporting the Tetsu Tetsu no Mi.