What is the source of the belief that the seventh son of a seventh son is a clairvoyant?

Question

The Iron Maiden song Seventh Son of a Seventh Son was inspired - according to Steve Harris - by the folklore belief that the seventh son of a seventh son has clairvoyant powers:

“I just had a thought: ‘I wonder if she could foresee her own death?’” stated Steve Harris, in 2013’s Maiden England ’88 documentary. “Who knows? So I started off with that sort of idea. I wrote The Clairvoyant and then went to Bruce with it and basically he said, ‘Yeah, it’s a great idea!’ I started then having an idea for a song, Seventh Son Of A Seventh Son, because supposedly if you were born the seventh son of a seventh son you had the powers of a clairvoyant. So I had those two ideas and Bruce went, ‘You know what? We should do a concept album about this…’”

_{Source: How Seventh Son Of A Seventh Son lifted Iron Maiden to heavy metal immortality. Dom Lawson. Metal Hammer.}

Wikipedia points to Ireland as the source of the belief that the seventh son of a seventh son has healing powers, something that is mentioned in the song:

Here the birth from an unbroken line
Born the healer the seventh, his time
Unknowingly blessed and as his life unfolds
Slowly unveiling the power he holds

...

Today is born the seventh one
Born of woman the seventh son
And he in turn of a seventh son
He has the power to heal
He has the gift of the second sight
He is the chosen one
So it shall be written
So it shall be done

However, I wasn't able to find where the belief that the seventh son of a seventh son has "the gift of the second sight" comes from.

Help?

Answer 1

The Chambers Book of Days (1864) has a passage about seventh sons and it's also referenced in Folk-Lore of Women(1906, and misleading title, it's very Victorian in its attitudes, as sacred-texts says on their index). It's an old superstition, obviously well entrenched by the mid-1800s, but the only real 'source' I've found is general stuff about the number seven being important across multiple cultures, etc etc.

HOWEVER I did some digging, and found an apocryphal Bible verse, which seems to me a better source for something like this than most. Namely, Testament of Dan which is about anger, and Fourth Book of Maccabees, which has a lot of sevens in it. Seven brothers, sevenfold companionship, seven days of creation, 'seven-towered right Reason', a lot of emphasis is put on the number. As for the clairvoyancy part,

Blockquote 11 And then standing on the brink of death he said, 'I am no renegade to the witness borne by my brethren. 12 And I call upon the God of my fathers to be merciful unto my nation. 13 And thee will he Punish both in this present life and after that thou art dead.' 14 And with this prayer he cast himself into the red-hot brazier, and so gave up the ghost.

I think this might be the closest we have to a source for this. It's vague and a bit of a stretch, but it's suitably ancient, and frankly over a few centuries even a little thing can get pulled out of proportion, so a little pseudo-prophecy before a self-sacrifice that's played as being noble isn't terribly unreasonable.