In Icelandic we have sunka, which means fall down, a sinking movement. It's more like the spelled sinkwan. I sank into the chair means. Ég sunkaðist ofan í stólinn or Ég sökk ofan í stólinn. But sökkva, synkva and sinkwan may have sounded similiar. Who knows?
Amikeco, does the German ü corresponds to the Danish y?
Could Gunther correspond to the Icelandic Gunnþór (Battle Thor)?