(Chapter 18) Footnote 45:

See note 30 above. We are unable to find evidence that the ancients taught their sons anything of the kind, but the fanciful notion put forward here by Vesalius persists in modern dictionaries, e.g., the 1989 Oxford English Dictionary (“so called because in man it was supposed to resemble the bill of the cuckoo”) and the 1992 American Heritage Dictionary. The coccyx bears little resemblance to the slender beak of the cuckoo, which is nearly straight. But if on Pliny’s authority Vesalius took the cuckoo to belong to the hawk family, he may have supposed the heavy, inwardly curved shape of the coccyx to resemble a raptor’s beak.