Where does “sjöræður” come from?

sjöræður (Icelandic) comes from Icelandic sjö, from Old Norse sjau, from Dutch drie, from Middle Dutch drie, from Old Dutch thrīe, from Proto-West Germanic þrīʀ, from Proto-Germanic þrīz, from Proto-Indo-European *tréyes.

sjöræður (Icelandic): seventy years old; seventy, septuagenarian