The seventh son of a seventh son is a concept from folklore regarding special powers given to, or held by, such a son. The seventh son must come from an unbroken line with no female siblings born between and, in turn, be born to such a seventh son.[1] The number seven has a long history of mystical and biblical significance: seven virtues, seven deadly sins, Seven Sleepers, seven days of creation, etc. In this case, it refers to a man who is the seventh son of a man who is himself a seventh son.
In some beliefs, the special powers are inborn, inherited simply by virtue of his birth order; in others the powers are granted to him by God or the gods because of his birth order.
https://en.wikipedia.org/wiki/Seventh_son_of_a_seventh_son
Seven seems to be a magical number in many cultures, and is often imbued with mystical and religious attributes. In the Abrahamic faiths, for instance, it is believed that God created the world in seven days, while in Greek mythology, the Pleiades were seven sisters who were the companions of the goddess Artemis.
Other groups of seven include the Seven Wonders of the World , the Seven Sages of Greece, and the Shichi Fukujin (Seven Gods of Fortune) of Japanese mythology . In folklore, seven also has a special role in the order of birth.
In European folklore, the seventh son of a seventh son is believed to possess special powers. The seventh son must be preceded by six brothers, with no sisters born in between, and whose father is also such a seventh son. Such a child is said to be gifted with the power to heal diseases .
Keeping a Seventh Son Tradition Alive
In 1907, another (possibly associated) custom about the seventh son was created. According to this custom, which originated in Germany, the reigning prince would be the sponsor to a seventh son of any of his subjects. When Enrique Brost and Apolonia Holmann, who were Volga Germans from south eastern Russia, immigrated to Argentina in the early 1900s, they brought the custom along.
They requested the then Argentinian president, Jose Figueroa Alcorta, be their seventh son’s godfather. The president agreed to their request and this tradition has been kept till today. In 2014, Yair Tawil, a seventh son, became the godson of Argentina’s president , Cristina Fernandez de Kirchner.
https://www.ancient-origins.net/myths-legends/legendary-powers-seventh-son-seventh-son-002794