皇こう太たい孫そんこうたいそんMeanings1nouneldest grandson of an emperor in the line of successionA formal imperial title; used in historical and modern contexts for the eldest grandson of the reigning emperor who is heir apparent.皇こう太たい孫そんは天皇てんのうの長ちょう男なんの長ちょう男なんにあたる。