The Memel is a river in Poland, in an area that used to belong to Prussia called Memeland. The song is supposed to come from this area. However, there seems to be some connection with Pete Seeger's "Where Have All the Flowers Gone". He says he wrote it starting from a couple of lines from a Cossack song quoted in a Russian novel ("And Quiet Flows the Don"? I forget.) The structures of the songs are similar, and it seems quite possible that the same Cossack song was picked up by someone from the Memelland. Conjecture, though!