The song, which has numerous variants, is a traditional Scottish ballad that dates from at least 1775. You can find more information about it at Wikiwand.
(as sung on this recording… Nathan does also sing it in an older dialect that is slightly different)
The king has been a prisoner
And a prisoner long in Spain,
And Willie of the Winsbury
Has lain long with his daughter at home.
“What ails you, what ails you, my daughter Janet,
Why you look so pale and wan?
Oh, have you had any sore sickness
Or yet been sleeping with a man?”
“I have not had any sore sickness,
Nor yet been sleeping with a man.
It is for you, my father dear,
For biding so long in Spain.”
“Cast off, cast off your berry-brown gown,
You stand naked upon the stone,
That I may know you by your shape,
If you be a maiden or no.”
And she’s cast off her berry-brown gown
She stood naked upon the stone
Her apron was low and her haunches were round,
Her face was pale and wan.
“Oh, was it with a lord or a duke or a knight,
Or a man of birth and fame,
Or was it with one of my serving men
That’s lately come out of Spain?”
“It wasn’t with a lord, nor a duke or a knight,
Nor a man of birth and fame,
But it was with Willie of Winsbury,
I could bide no longer alone.”
And the king has called on his merry men all,
By thirty and by three,
Says, “Fetch me this Willie of Winsbury,
For hanged he shall be.”
But when he came the king before,
He was clad all in the red silk.
His hair was like the strands of gold,
His skin was as white as the milk.
And “It is no wonder,” said the king,
“That my daughter’s love you did win.
For if I was a woman, as I am a man,
My bedfellow you would have been.”
“And will you marry my daughter Janet,
By the truth of your right hand?
Oh, will you marry my daughter Janet?
I will make you the lord of my land.”
“Oh yes, I will marry your daughter Janet.
By the truth of my right hand.
Why yes, I will marry your daughter Janet,
But I’ll not be the lord of your land.”
And he’s mounted her on a milk-white steed,
And himself on a dapple grey.
He has made her the lady of as much land
As she shall ride in a long summer’s day.