Bastardized form of the word "mukluk" which refers to a boot made from animal skin worn by Eskimos or boots of similar fashion. Yeah, this is the word Krusty uses in that particular Simpsons episode.
Krusty: Kent, the young people today, they think comedy is dirty words. It's not -- it's words that *sound* dirty, like "mukluk."
Alright, so you go to McDonald's and you take a look at the dollar menu, order a Double Cheeseburger and a McChicken. After receiving your order, take the Double Cheeseburger and split it in half, then put the whole McChicken in the middle and reassemble. You are now ready to enjoy a McCluckin.