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 be, in turn, born to such a seventh son. The number seven has a long history of mystical and religious associations: seven virtues, seven deadly sins, seven Sleepers, seven-league boots, seven ages of man, seven days of creation, seven hills of Rome, seven lucky gods of Japanese mythology, the Seven Sages, seven sisters, seven stars, seven wonders of the world, 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.