Helljumpers I'd the nickname for the ODST (Orbital Drop Shock Troopers), they are a special forces group for the UNSC (United Nations Space Command). They get the nickname helljumpers because the drop from ships in low orbit of a planet, this involves entry into the planets atmosphere. Bacisly what spaceships go through when entering Earths atmosphere but in a pod like craft they size of a single person. Their nickname also comes from their motto "feet first into hell".
Marine 1: What the fuck is that!
Marine 2: What the fuck are you talkimg about?
Marine 1: Those things in the sky.
Marine 2: Oh shit Hellsljumpers! We are going to kick some serious as now!
Marine 1: Who are Helljumpers?
Marine 2: The best fighting force in the known universe.
Someone who's amazing at a certain thing; taken from the actual drummer with the coined nickname "Hellhammer" From the Black Metal band 'Mayhem', or in the drumming world, someone who pounds the drums incredibly fast and keeps rhythm and consistancy.
"Jesus, it was over in a second, he just came up and hellhammered that punk."