Urchin is the old English term for hedgehog. As such, it is applied to many things that take a similar form to a hedgehog:
- Sea urchins are round and prickly like hedgehogs.
- Children (especially boys) that behave in an annoying manner are commonly called urchins. This comes from the scrunched-face appearance of both hedgehogs and unruly children.
- In old theater, urchin was a common name for an elf or fairy that took on a prickly appearance.
For a detailed etymology, see http://www.etymonline.com/index.php?term=urchin
