The Collaborative International Dictionary of English v.0.48:

Coccyx \Coc"cyx\, n.; pl. L. Coccyges. [L., cuckoo, Gr. ?,
   cuckoo, coccyx. So called from its resemblance to the beak of
   a cuckoo.] (Anat.)
   The end of the vertebral column beyond the sacrum in man and
   tailless monkeys. It is composed of several vertebr[ae] more
   or less consolidated.
   [1913 Webster]

WordNet (r) 3.0 (2006):

coccyx
    n 1: the end of the vertebral column in humans and tailless apes
         [syn: coccyx, tail bone]