Here are the lyrics as Eliza Carthy sings them (and pretty much as Martin Carthy sings them). It's a bit different from the other versions that have been posted, with one particular difference that I want to ask about, below.

Eliza Carthy sings Billy Boy
https://mainlynorfolk.info/martin.carthy/songs/billyboy.html

“Where have you been all the day, bonny boy, Billy Boy?
Where have you been all the day, o my dear darling Billy O?”
“I've been out all the day
Walking with a lady gay,
Isn't she a young thing lately from her mummy O?”

“Is she fit for your wife, bonny boy, Billy Boy?
Is she fit for your wife, o my dear darling Billy O?”
“She's as fit to be my wife
As the heart is to the knife,
Isn't she a young thing lately from her mummy O?”

“And did she ask you to sit down, bonny boy, Billy Boy?
Did she ask you to sit down, o my dear darling Billy O?”
“Well she asked me to sit down
Then she curtsied to the ground,
Isn't she a young thing lately from her mummy O?”

“Did she light you up to bed, bonny boy, Billy Boy?
Did she light you up to bed, o my dear darling Billy O?”
“Well she lit me up to bed
With a nodding of her head,
Isn't she a young thing lately from her mummy O?”

“Did she lie close to you, bonny boy, Billy Boy?
Did she lie close to you, o my dear darling Billy O?”
“Well she lay so close to me
As the bark is to the tree,
Isn't she a young thing lately from her mummy O?”

“Do you want to know her age, bonny boy, Billy Boy?
Do you want to know her age, o my dear darling Billy O?”
“She is twice six seven,
She is twice twenty and eleven,
Isn't she a young thing lately from her mummy O?”

There's also a nice recording by Luke Plumb and Kare Burke on their new album.

Now, verse 2, lines 3 and 4: She's as fit to be my wife/As the heart is to the knife.

What is going on there? How is a heart fit to a knife? Other versions have fork and knife, or sheath and knife, which make sense to me. Is heart and knife a mondegreen?