The "healthy" alternative to a "burger" or chicken clumps; a sandwich comprised of some sort of a fried fishy patty on a bun with an enormous dollop of mayo-AYO, available at McDonald's.
To insert your entire hand into the vagina and/or anus of another whilst holding a McDonald's hamburger sandwich, most often a BIG mac but in essence, any hamburger sandwich from McDonald's extensive menu would constitute the act of McFisty.