Baha, I finally find out! Is it just rattled off in one word like a curse/epithet?
Yes, one word, but not necessarily a curse. It is just a normal word, though you could easily use any word as curse in the right mood (Just try calling your neighbor "You cute little pony" in such a situation - more scary is hard to achieve).
The root of the word is simple "wumpe", which means "irrelevant, unimportant, doesn't matter" in the local slang - in southern Germany, it failed to survive the consonant shift I fear, I think it is also unknown in the Ruhr area. But everything formerly Prussian understands this word, it seems to be from Berlin by some theories. But its new home is around Braunschweig.
If you want to know more about the origin of the word, here is a nice German article about failing to find this origin*:
http://www.sueddeutsche.de/service/sprachlabor-was-bedeutet-das-adverb-wumpe-1.962930
It just appeared by divine providence I fear.
If it is really irrelevant and just a delaying factor, you could strengthing the word. In this case, by prefixing "Ur-" to it, another german language dinosaur, like used in "Uralt" (very old), "Urschnell" (very fast) and especially "Urwichtig" (very important) - whichs opposite would be "Urwumpe".
* But it contains a lengthy praise of its phonetic qualities, that I can only agree with. On par with Tüddelkram.