It is 10-4, known to many of us who grew up in the 50's as "Broderick Crawford Day."
For younger readers, Broderick Crawford starred as Dan Matthews in Highway Patrol a really cheesy (and, since this was the 50's, that's saying something) police chase TV show. The opening of the show is below:
There are many blogs that celebrate this day. Go here, here, and here for samples.
How cheesy was it? A show about car chases was sponsored by a brewer!
Here is a photo of the man himself in action. He said, "10-4" more than he said "hello," "I" or "let's get some donuts."
The show was so popular that he reprised his role on an episode of CHiPs. He says the magic word at the 3:16 mark.