Okay. I figured out what was wrong.

Here's how it's supposed to go:

First person: What's the word?
Second person: Mockin bird.

**

And of course, "What's the word?" means the same thing as "What's happenin?" and "What's the news?" and "What's goin on?"

But "What's goin on?" beats all the rest in the song department, thanks to Marvin Gaye's soulfully deep take on this colloquial expression:

http://www.youtube.com/watch?v=kiZC_WrCqKk