Specifically, "grue" means "green before January 1, 2000 and blue on or after January 1, 2000". See also bleen.
A grue is also a fictional predator from the dungeon-adventure game Zork, and the Zork series of interactive fiction games by Infocom.
The grue, according to the scholars of the Great Underground Empire, is a sinister, lurking presence in the dark places of the earth. Their favorite diet is adventurer (i.e. the main character of the game), but their appetite is tempered by their intense fear of light. No Grues have ever been seen by the light of day, and even a feeble torch is sufficient to frighten them away in their underground lairs.
Of those who have seen grues, few have survived their fearsome jaws to tell the tale. Grues have sharp claws and fangs, and an uncontrollable tendency to slaver and gurgle. They are certainly the most evil-tempered of all creatures in the Zork milieu; to say they are touchy is a dangerous understatement. The expression "Sour as a grue" is common even among grues themselves.
The existence of grues was used by game designers to control player behavior by limiting light sources.
According to the Jargon File, Dave Lebling took the name from Jack Vance's "Dying Earth" fantasies, and the word is an archaic English verb meaning to shudder with fear (as in the still-current word "gruesome").