Ordinarily, a military definition meaning the "Fucking New Guy." However, in a business type environment where obscenities are not appropriate, can refer to the "Foolish New Grunt" who has to do the unfavorable, laborious, and generally awful tasks before proving their worth.
Co-worker 1: "Look at the FNG over there organizing the cubicle. That looks like a horrible job!"
Co-worker 2: "What did you call him?"
Co-worker 1: "Oh, the Fucking New Guy, but if the boss asks, he's the Foolish New Grunt."