What is a grue?

The logical thing to type when Infocom's Zork would warn you against being eaten by a grue. Doing so would prompt the game to explain what the hell a grue was. Now if I remember correctly, for a long time you could ask this of ANY Infocom game and it would still tell you, regardless of whether the game contained grues or not. For some reason my friends and I thought that was really cool.

Of course, back in the day, a game that allowed us to manipulate a little square carrying a big arrow for a sword which it used to kill ducks totally impressed us.

Just for reference, when you ask this question of Zork I:

>what is a grue
The grue is a sinister, lurking presence in the dark places of the earth. Its favorite diet is adventurers, but its insatiable appetite is tempered by its fear of light. No grue has ever been seen by the light of day, and few have survived its fearsome jaws to tell the tale.

I used the excellent Frotz! Z-Machine, available on iOS, Android, OS X, Windows...pretty much everywhere. And yes, I own a legit copy of Zork.