Could it be any relation to a "warmus"? When you say the word outloud, it sounds kinda like "wampus."http://books.google.com/books?id=ri8WAAAAYAAJ&pg=PA254
The "warmus" is a working garment, similar in appearance to a "roundabout," but more full, and being usually made of red flannel, is elastic and easy to the wearer. It is an article generally unknown in New England, New York, and the extreme northern or southern part of our country, but is more peculiar to the Germans of Pennsylvania. If any traveller, in passing through Ohio, should chance to see a large number of "lobster back" people on the farms, or about the village taverns, he may at once know, without any inquiry, that he is among the descendants of the worthy settlers of the "key-stone state."
That was 1847, so they may have spread later, as German influence spread in general. Might be worth checking out.