The cry is "Whojackie" and an explanation of its origins is given here by Ethan Suplee (Randy Hickey)...
"...years ago, apparently, on Roseanne, she [Roseanne Barr] would just hire her friends to be writers on that show. There were legit writers too but she'd come in and say this is my friend Soandso and he's gonna be a writer. Eventually there were a few of these guys and not all of them were that familiar with the show. Anyway, one day they were talking about a b-storyline with the Jackie character [Roseanne's sister in the show] and one of these "writers" that was a friend of Roseanne's said "whojackie?" because he didn't know who that was. I don't know if that actually happened but that's the story I heard and the reason behind "whojackie."
http://www.nbc.com/My_Name_Is_Earl/ethans-insi der-blog/
WhoJackie was also used in the show as the online screenname of Josh, the guy Joy accidently kidnapped when she stole a truck and who later died in an unfortunate bed-crushing accident. Posts by this user were set up on the My Name is Earl forum of TWoP as a tie-in with the show and here is Joy's post from the funeral episode...
http://forums.televisionwithoutpity.com/index. php?showtopic=3129222&st=2748