"What does it mean by 'speak, friend, and enter'?" asked Merry.
"That is plain enough," said Gimly. "If you are a friend, speak the password, and the doors will open, and you can enter."...
"But do not you know the word, Gandalf?" asked Boromir in surprise.
"No!" said the wizard..."I do not know the word - yet. But we shall soon see."...
Picking up his staff he stood before the rock and said in a clear voice: Mellon!
The star shone out briefly and faded again. Then silently a great doorway was outlined, though not a crack or joint had been visible before. Slowly it divided in the middle and swung outwards inch by inch, until both doors lay back against the wall. Through the opening a shadowy stair could be seen climbing steeply up; but beyond the lower steps the darkness was deeper than the night. The Company stared in wonder.
"I was wrong after all," said Gandalf, "and Gimli too. Merry, of all people, was on the right track. The opening word was inscribed on the archway all the time! The translation should have been: Say 'Friend' and enter. I had only to speak the Elvish work for friend and the doors opened. Quite simple. Too simple for a learned lore master in these suspicious days. Those were happier times. Now let us go!"
Lord of the Rings