The MSG sword is the name of the "unbreakable" Master sword that you get before it breaks. Explanation in comments.

The MSG sword is the name of the Master sword you get when you're exploring the ruins with Zelda. With a glitch, you could actually transport the sword from a new game to a game where you completed the tutorial. The name can be accessed by transferring a weapon to the new game save and picking it up would allow you to access the weapons section early and the MSG Sword is what it was called. The glitch could only be done on earlier versions of the game which was the point of the joke.

On YouTube, look up Suishi, he is the one who will explain the glitch.

Technically by unbreakable, it means that every time you use the sword to hit something, it doesn't take off it's durability. It was coded to do that.