From Schwéier- + Papp. The first element may be either Middle High German swiger (“mother-in-law”) or sweher (“father-in-law”), from Proto-Germanic *swegrō and *swehuraz respectively. While German generalised the feminine in Schwiegervater (displacing obsolete Schwähervater), the Luxembourgish development may have been the other way round, because only Middle High German sweher regularly yields Schwéier. From swiger one should expect either *Schwiger or *Schweier. However, it is also possible that these were altered after Schwéiesch (“sister-in-law”), which derives from yet another related word, namely Proto-Germanic *swēgraz (“brother-in-law”).
Schwéierpapp m (plural Schwéierpappen, feminine Schwéiermamm)