coccyx n.

"part of the human spinal column consisting of the last four bones," 1610s, from Latin coccyx, from Greek kokkyx "cuckoo" (from kokku, like the bird's English name echoic of its cry), so called by ancient Greek physician Galen because the stunted, coalesced tail-bones in humans supposedly resemble a cuckoo's moderately curved beak. Related: Coccygeal.