MLA for Rear Echelon Mother Fucker. A derogatory term of US Vietnam era origin that's somehow found it's way in to the lexicon of British Army slang - probably due to repeated showings of Chicken Burger Ridge in the NAAFI. Anyone who is further from the FEBA than the person using the phrase is a REMF - the bloke 'in the rear with the gear'. Generally, if you are so far back that your laundry goes forward, you are a REMF.
That said, in today's operational environment even REMFs are on the front line: IEDs, rockets and mortar rounds making no distinction between infantryman and hygiene operative.