The secret walnut tree or ship's keel?

That's just the meaning in Greek. In the modern ISV (International Scientific Vocabulary) caryo / carya means something more like 'nut, kernel' (caryopsis) or 'cell nucleus' (karyogamy). I guess it's kind of the opposite of gymnosperm 'naked' + 'seed'.

The ship's keel had me going for a while until I realized it was the tree that provide the piece for the keel and not the tree-nuts.

Ceci n'est pas un seing.