The Copper Key was the first of the three keys needed in the quest to find Halliday's Hidden Egg. It was first mentioned in Anorak’s Invitation. It is inscribed with the following clue to discover the First Gate:
What you seek lies hidden in the trash on the deepest level of Daggorath.
This clue is on the Key.
The Copper Key is located on the planet Ludus, within the Tomb of Horrors; to earn the key, the character must defeat Acererak the Demi-Lich in two out of three games of the arcade game, Joust, which Wade practiced with Aech
The key is used to open the first gate which takes Wade Watts into a copy of the movie "War Games" where Wade has to play through the movie as the main character reciting all the dialog to earn points which contribute to his score on the leader board.
The clue to the Copper Key was hidden within Anorak's Almanac. When an MIT freshman named Steven Pendergast announced to the media that he had discovered the clue (even though Wade Watts himself had secretly discovered 6 months previously) newsfeed broadcast interviews with him for a month despite him having no idea what the clue could allude too. After this incident, going public with a clue was known as "pulling a Pendergast".
Four years after the clue became public knowledge (thanks to Pendergast's announcement), the clue had become known as "the Limerick" by gunters.
In the movie, the key is received by winning the first challenge, a dangerous race through New York City to Central Park. After Parzival wins the race, he is given the key by Anorak and the clue to the second challenge rises out of a nearby fire hydrant. He is followed soon after by Art3mis, Aech, Daito and Sho. When Parzival wins the Copper Key, 100,000 coins are also deposited into his account.
At the end of the movie, after getting the Crystal Key, Parzival inserts the Copper Key into the bottom of the three locks that are on the gate that appears.