Eupen.

First attested as Oipen in 1213. From Middle Dutch Oepen, Oipen, Oupen, Open. According to the prevalent theory from Proto-West Germanic *opan (“open”). The Middle Dutch spellings indeed seem to point to a back vowel; on the other hand there is no certainty about this and the modern front-rounded vowel would remain unexplained. An alternative theory derives it from *up (“up, at the top”) + *-īn. This explains the umlaut, but is still at odds with the dialectal pronunciation [ˈøˑʏ̯pə] ~ [ˈøːpə], which would require West-Germanic au-. Therefore a third theory postulates an underlying *Aupīn, in which *aup- would be a blend of *auwju (“floodplain”) with the hydronym *apa-. The spelling in Eu- appears in the 16th century and is probably a Franco-Flemish representation of [øː]. The contemporary Standard German pronunciation may to some degree be a spelling pronunciation, but it also approaches the local [øˑʏ̯], which moreover generally corresponds to standard eu, äu.