Osnabruck is a German city, located in Lower Saxony. An "osnabruck" must have been a slang term for some sort of garment associated with or originating from Osnabruck (sort of like a Panama hat). It's anybody's guess what this might have been, although a social history of the mid-nineteenth century might know what the term meant.
It looks like you the meaning right. Osnabrück was a North German town known for its manufacture of coarse linens. "Osnabrucks, "osenbreges," "ostenbriges," "osnabrigs" "ozenbrigs" were various terms for the cloth and came to be associated with the trousers or pants that might be made of them. But also, "britches" or "breeches" came to mean "trousers" as well, as did "broek" in Dutch and "Hose" in German, so there's a strange connection between the name of the town and clothing.