As I recall, vindelis is correct and the 7th son of a 7th son is supposed to have "second sight".On the musical side (if it can be called music) I seem to remember Bob Hope (and I assume Bing Crosby) singing about the 7th son of a 7th son (and committing appalling "cod" Scots accents) in (I think) one of the "Road" series of films (no idea which one).
Regards
Walrus