The term comes from how slow Mississippi residents are in the head. It takes them forever to figure out how to do even the simplest task. Not only are they dumb, but they also can't move fast because their fat rolls are in the way of their feet...and eyes.
If you didn't know Mississippi has been voted one of the top dumbest states and has been the fattest state in the U.S. for three years straight now.
Jim Bob: "Sure man, ill be there in a Mississippi Moment."
John: "Thanks man!"
(4 hours later)
John: "WHERE THE HELL ARE YOU?!?!"
Jim Bob: "Home."