I come across with this expression every once in a while.
I tried some english dictionaries and web encyclopedias, to no avail.
I tried asking about it to a few customers at work, and to my surprise, they didn't seem to have much idea either. I mean, I've seen it at newspapers, web articles, and elsewhere I now can't remember. I thought it was a widely-known expression.
And it seems like the expression is used in two different manners. As a noun, like ho-hum, or as an interjection of some sort.