So a small (49 seat) burger joint in Edmonton, called Woodshed Burgers, created a cod-burger which they called the Effing Filet O'Fish.
Mcdonalds, purveyor of probably 49 trillion burgers, felt that this threatened the global market dominance of their Filet-o-fish burger. So they threatened legal action.
Woodshed Burgers, not having the resources to hire more than 49 lawyers to fight the case, wisely backed down and changed the name of their burger.
To the McEffing Fish Fillet.