A Rear Echelon Motherfucker REMF is someone who has a relatively safe job and good living conditions in a war zone who is hated (because of envy) by frontline combat soldiers who have neither, and wished they did.
There are two basic types of REMFs: Pencil pushing administrators, and combat support personnel. It is a myth that REMFs risk the lives of those in combat on the frontlines, in fact the opposite is true. Combat support REMFs keep the grunts alive by supplying "beans, bullets and blood."
The FNG Army PFC 11B was pissed off at the personnel clerk REMF at the 20th Replacement Center at Long Binh for assigning him orders him to 1st Infantry at Khe Sahn to help the Marines fight the NVA. When besieged Khe Sahn was resupplied by C-123 airdrops of ammo, medical supplies and food loaded by REMF Air Force combat support troops at Quang Tri & Da Nang, the Army PFC 11B realized he had been hating out of envy the guys he owed his life to. Some of which later were KIA in rocket attacks.