Couple things I don't see mentioned above, (~pace~ Dave's Wife)tho they may well enough have no bearing -

I remember my dad telling me (from his experience in the area during the large army manoeuvers in Louisiana around 1941), that there was then still enough high feeling between the Italian & Corsican immigrants and the Powers-That-Was that you could count on starting a riot by going to the right neighborhood and yelling "Who kill-a da Police?". Seems as if one interpretation might be a creolization of of something like "Giacomo fini" or "Giacomo fait fini", intended as a taunt & challenge, and referring back to the troubles at the beginning of last century, and meaning something like either "Nyah, nyah, Dago Jake is done for" or "Ha, Jake finished (whomever)".

Also, any possibility of "Iko" being a variant/derivative of "Iku"? I have a set of "Tarot" cards modeled on the parallels with the Santerian Orishas, wherein "Death" is "Iku", apparently from Yoruba. "Death gonna get you" sounds like a good threat.

My personal $.02 is that Yanne's derivation from Creole looks the most persuasive, but in the linguistic gumbo of N'awlinz, probably you could find a dozen different Indians who chanted it at different times and in different ways who would all swear it meant something different. Obviously "Kiss my ass" and "Eat my shorts" and "I'm gonna **** you up" are not the literal translations, but they look like it feels, as you might say. Form follows function, remember?