Short form for "Just Kidding, United States Federal Bureau of Investigation Agents".
When you are typing something online, when you type something about something that was or is illegal, you follow it up by saying "jk feds". Since everyone knows the feds moniter everything on the net, you can cover your ass by "jk feds" when you say something illegal.
"I just sold that cat an 8-ball of bolivian flake....jk feds"
You cover your ass by telling the feds your joking, but really your not.