In science fiction, there are many mottos and catch-phrases... shibboleths endlessly repeated by fans, sometimes ironically, sometimes in earnest. "May the Force Be With You," for example, or "To boldly go where no man has gone before." Those phrases have been burned upon the genre's collective consciousness through endless conditioning and repetition, a Ludovico technique interpreted and applied by Hollywood marketers.

But there's another phrase blinking in our cultural memory banks -- a babble of alien phonemes shouted to stop a silver humanoid robot from destroying the planet Earth. The phrase only appeared in a single movie that never got a sequel, but it has been endlessly repeated through the genre as code phrase and callback. Frederick S. Clarke has called it "the most famous phrase ever spoken by an extraterrestrial..." -- "Klaatu barada nikto!"

The phrase has no actual meaning, but in its original appearance -- 1951's The Day The Earth Stood Still -- it has a very specific purpose. The alien Klaatu is sent to Earth to warn humans to give up their violent, war-like ways, lest it spill into the stars. Klaatu is accompanied by a robot named Gort, from a race of bio-mechanical golems that act as ultra-violent, intergalactic peace keepers. Of course, humans being what they are, they take a shot at Klaatu, triggering Gort's kill switch. It is only the phrase "Klaatu barada nikto!" that stops the robot from creaming Earth. There's no direct translation of the phrase, but since the first word is the name of Gort's master, clearly it's a command of some kind: "Klaatu says stop monkeying around" or some such. Whatever the actual meaning, having heard the safe word, Gort collects Klaatu and takes their shimmering saucer into space, where Klaatu is miraculously revived just long enough to tediously moralize about the savagery of man. The End.

"Klaatu barada nikto" may have gone down as just another spurting of vaguely alien-sounding gobbledygook if the film had been released in another era, but The Day The Earth Stood Still -- boring and preachy as it may seem now -- was an important picture in the genre. It was one of the first "serious" science fiction films and one of the first films to treat aliens as the moral superiors of man. These facts doubtlessly helped "Klaatu barada nikto!" enjoy a larger life outside of its initial narrow scope.

And the larger life of the phrase is huge. Wikipedia's entry on "Klaatu barada nikto" mentions hundreds of references in books, comics, film, television and music. Including:

• In Sam Raimi's Army of Darkness, Ash is told that the phrase "Klaatu barada nikto!" must be repeated before he takes the Book of the Dead, but forgets how to pronounce the last word and tries to "fool" the Deadites by coughing over it, triggering a medieval zombie apocalypse.

• In Return of the Jedi, there are two aliens on Jabba the Hutt's Sail Barge who reference The Day the Earth Stood Still: Klaatu, an alien of the Nikto species, and Barada, an alien of the Klatooinian race.

• Typing "about:robots" into the address bar of the Firefox browser brings open a page named "Klaatu barada nikto!" that references many famous robots.

And so on. Worryingly, Keanu Reeves will soon repeat the famous robot-soothing command in this year's remake of A Day The Earth Stood Still, a nonsensical update that changes Klaatu's mission to earth to an environmental one: Aliens, it appears, are passionately concerned with our carbon credits, and will steamroll us with an armada of rampaging killbots if we don't recycle. The concept bodes ill, but watching the mush-mouthed Keanu try to wrap his tongue around the phrase will probably be delightful, in a hilariously awful sort of way.

In regards to placating killer robots, science fiction is pretty clear: If you can't get its head to explode by forcing it to consider the nature of the human emotion called "love," you can always try shouting "Klaatu barada nikto!" at the top of your lungs. Any command with at least the potential of stopping a robot from stomping your spine into goo is worthy of its own entry in our dictionary of scifi.