An acronym, short for "Modifications and Additions to Reactor Facilities." It is the name of a prototype reactor plant run by the U.S. Navy and the Department of Energy, located in upstate New York. It is a disastrous cluster-fuck of a Destroyer's engine room (surface ship's steam plant) and a S5W/S7G primary plant (fast-attack submarine's reactor plant), which results in nothing making any sense at all, and everything breaking constantly. According to a rumor I just heard, a large primary-to-secondary leak recently occurred and the plant is going to be shut down forever.
Also means, "Make another round, fucker!" and is said to someone who's sitting around, on watch, not using their time the way they're supposed to (bullshitting, not paying attention to equipment/machinery).
Person A: What plant are you in? S8G?
Person B: MARF :(
Person A: *shivers* I'm sorry man.