A burp accompanied by an unintended reflux of stomach contents. The word is a portmanteau of chunder and burp, and may be considered the oral analogue of a shart.
Sandy: Reuben, I'm in a situation here. We have to leave now!
Reuben: No. Can we stay a couple more minutes?
Sandy: Dude, no. This is serious. I just churped.
Reuben: I don't know what that means.
Sandy: I tried to burp and a little chunder came out - I just churped. Now let's go!