I never found out what kind of lady a "Wetfru" is. "De junge Wetfru", which I learned from Bernd Gerken, is a sad little song of just three verses about lost love which he also sang in the Idle Fellows
Since writing the above in 1998, I've finally (July 1999) learned from surfer Kathy Bartel that a "Wetfru" is a widow. Thanks a lot Kathy!
Surfer Anja Meyfarth wrote in December, 2001 that the song was written by Klaus Groth, the most famous poet of Low Saxon language. He wrote the song in troublesome times, when the people of Slesvig-Holstein fought against the Danes in 1848. It describes the feelings of a young widow who lost her husband. The song is one of Anja's favourites. Many thanks Anja!