Victor Mair had a recent post at the Log in which he discussed some bits of spoken Peking Chinese that have been mashed into unintelligibility (if you’re not part of the in-group):
This afternoon I passed by a group of high school kids from China going down the street outside of Williams Hall, the office building in which I work. One of the girls said merrily, “Bur’ao”, by which she meant Modern Standard Mandarin (MSM) bù zhīdào 不知道 (“[I] don’t know”).
The retroflex final -r is well known for northern varieties of Mandarin, but in Pekingese it seems that the mighty R has the ability to swallow up whole syllables, as in the example quoted in the previous paragraph.
He provides a number of other examples (not all involving -r), and in the comments he adduces the English parallel “sup,” which he heard in a bar full of sailors: “They were all giving high-fives to each other and saying that. I had absolutely no idea what it meant. I knew that it must be something very common in their English (in fact, it was the most frequently uttered expression in that bar), but I felt so silly not being able to figure out what such a common expression meant. [...] It took me several tries before I found someone who was patient enough to explain to me that it meant ‘What’s up?’”
I suppose most languages must have such forms; in Russian, for instance, there’s “чё.”