(n, one word) Anyone whose primary assigned job tends to involve using a mop, or cleaning things. Often meant to degrade that person, especially when that person was actually hired or trained to do a much more complicated job. Used most frequently to describe Navy personnel (as an alternative to "swabbie") to whom this has happened, since it happens there with startlingly high frequency.
"The Navy spent 12 months training me to fix radars, then when I got to my boat, they sent me to the mess decks, and since I don't know how to cook, they made me a mopjockey. I get to swab decks all day. Whoopie. Glad I joined."