(Chapter 18) Footnote 31:

We would suppose because the flesh and ligaments make it resemble the cuckoo’s long tail, but Vesalius compares it to the bird’s beak at the end of this chapter. The cuckoo’s name is thought to be derived from the sound of its call, but the use of its name for the coccyx is elsewhere unexplained. Vesalius mistakenly cites Pliny Nat. Hist. 10 Ch. 9 in the margin. Pliny describes the bird and its habits in Nat. Hist. 10.25-27 (Ch. 11), wrongly identifying it as a species of hawk. The association of the cuckoo with silliness is at least as old as Aristophanes’ Nefelokokkugi/a, “Cloudcuckooville,” in the Birds (414 B.C.). A harbinger of spring (and perhaps therefore of spring silliness), it is obscurely associated with indecency in Nat. Hist. 18.249: Pliny says the sound of the cuckoo bird is imitated by people mocking farmers who prune their vines, “as it is considered disgraceful and deserving of reproach for that bird to find the pruning-hook being used on the vine.” This, says Pliny, is why obscene or scurrilous jokes (petulantiae sales) are thought to bring bad luck. The application of ko/kkuc to the bone that anchors several muscles to the anus is apparently Galenic, and may therefore be influenced by Lat. cacare, “defecate.”