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. The number has a long history of mystical and biblical significance: seven virtues, seven deadly sins, Seven Sleepers, Seven Heavens, etc. In this case, it refers to a man who is the seventh son of a man who is himself a seventh son.