People don't say exactly what they mean, and they don't mean exactly what they say. Slang changes, aphorisms abound, and the elephant in the room isn't spoken about. It's miraculous we understand each other at all, sometimes.
It's been said, and it's true, that the only times people speak in detail about something they do regularly (a job, a tradition, etc.) are when there's a newbie learning the ropes or when something has gone wrong with the system. Thus, the tradition of the main character being new to something so that it has to be explained to him and therefore the readers.
Everything else is an "As you know, Bob," even if those words are never used. Some writers are better at hiding it than others, and to some degree we've all been conditioned to accept expository dialogue as necessary... but as writers, we all want to do better.
You need to get information to the reader, and dialogue is one of your tools. You can bring a newbie into an established system, and you can use the disruption of the system as part of your plot. Here are a couple more thoughts/tricks I've found.
Context, context, context
You can use the natural tendency to not mention ordinary things to pique the reader's interest. Here's an off-the-cuff example from real life:
"You getting anything?"
"I was gonna kick it again."
This conversation is usually shouted from one end of my house to the other and ends with someone switching off the cable modem, counting to ten, and turning it back on. And that's really the most important part, because it's not clear dialogue. The reader thrown into it doesn't know what's being talked about, and will glom onto whatever you give them and assume it explains the dialogue.
If I were to write the above dialogue and then send somebody to the fridge to dish up some ice cream, the reader would try to make sense of that. And probably be quite confused.
Coding as a story hook
We all re-define words on the fly. Words pick up particular meanings in the context of any group of people -- your co-workers, your long-term relationship. your family. They become inside jokes, or reference important events in shorthand. And more importantly, once you've defined them you rarely explain that definition to anybody.
This can make coded dialogue powerful if the reader was there when the referenced events happened. But even if the readers don't know what happened, if you make it obvious that there's coding going on the readers will want to know. It's a hook.
How do you make it obvious? Body language, unexpected reactions, things like that. You've seen this done in movies and television, with varying degrees of subtlety, and you've seen it done in real life. How subtle you want to be depends on your readership, the genre, etc. -- this is something you want your beta readers to report back on whether it's working or not.
Bad grammar and dialect
People don't talk good, all the time. People have accents. Regionalisms. They lapse into other languages. This is a tricky area; yes, it will add color and texture and reality, but it also drives some readers straight up the wall. If the reader can't make heads or tails of what the character's saying for pages on end, they lose a reason to keep reading.
I was one of those people who just picked up A Clockwork Orange and plowed through it, language and all. Too stubborn to read the glossary. I would not expect my reader to let me get away with that sort of thing, though, so: moderation. Be careful. Poll your betas.