I think it might be a Danish expression. Brown soap is a strong soap containing lye, more likely sold near the things for cleaning floors and surfaces than you

I think it might be a Danish expression. Brown soap is a strong soap containing lye, more likely sold near the things for cleaning floors and surfaces than your body. https://da.wikipedia.org/wiki/Brun_s%C3%A6be https://da.wikipedia.org/wiki/Brun_s%C3%A6be