Someone who eats fast food frequently or over indulges the fast food experience; ordering specialty items, mixing condiments, requiring tedious preparation before dining, or preforming ritual or traditional tasks prior to consuming.
"Wow, that girl sure is a mcpiggins."
"Yes, notice the way she is piling copious amounts of their special sauce on the top of her food."
"My god, did she bring utensils from home?"
"Look, now she's arranging a massive collection of french fries in what appears to be a recreation of Golgatha."
"They will be forced to remove her from her seat using the Jaws of Life after inhaling all of that."
"Quite a McPiggins."