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.
I don't feelso good...I shouldn't have ordered that McFishwich for lunch!
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.
Boy oh boy, this hamburger is causing acute sexual arousal, bend over. It's McFisty time.