The word Yushi (猶子, the adopted child) means to treat like an Imperial Prince and it is believed to have been put into practice in the Imperial Family for the first time when "Shokugensho" (a book on Japanese government officials) asserted that Imperial Prince Tadafusa was the Yushi of Goudain (the Retired Emperor Gouda).
尚、猶子とは皇子に準じた扱いとする事だが、皇室について用いた始まりは『職原鈔』に忠房親王が後宇多院の猶子となす旨記された事に始まるという。