The Collaborative International Dictionary of English v.0.48:

Urchin \Ur"chin\ ([^u]"ch[i^]n), n. [OE. urchon, irchon, a
   hedgehog, OF. ire[,c]on, eri[,c]on, heri[,c]on, herichon, F.
   h['e]risson, a derivative fr. L. ericius, from er a hedgehog,
   for her; akin to Gr. chh`r. Cf. Herisson.]
   [1913 Webster]
   1. (Zool.) A hedgehog.
      [1913 Webster]

   2. (Zool.) A sea urchin. See Sea urchin.
      [1913 Webster]

   3. A mischievous elf supposed sometimes to take the form a
      hedgehog. "We 'll dress [them] like urchins, ouphes, and
      fairies." --Shak.
      [1913 Webster]

   4. A pert or roguish child; -- now commonly used only of a
      boy.
      [1913 Webster]

            And the urchins that stand with their thievish eyes
            Forever on watch ran off each with a prize. --W.
                                                  Howitt.
      [1913 Webster]

            You did indeed dissemble, you urchin you; but
            where's the girl that won't dissemble for an
            husband?                              --Goldsmith.
      [1913 Webster]

   5. One of a pair in a series of small card cylinders,
      arranged around a carding drum; -- so called from its
      fancied resemblance to the hedgehog. --Knight.
      [1913 Webster]

   Urchin fish (Zool.), a diodon.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Urchin \Ur"chin\, a.
   Rough; pricking; piercing. [R.] "Helping all urchin blasts."
   --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

urchin
    n 1: poor and often mischievous city child

Moby Thesaurus II by Grady Ward, 1.0:

64 Moby Thesaurus words for "urchin":
   Arab, JD, beach bum, beachcomber, beggar, bo, brat, bum, bummer,
   cub, dickens, dogie, elf, enfant terrible, gamin, gamine,
   guttersnipe, hobbledehoy, hobo, holy terror, homeless waif, idler,
   imp, juvenile delinquent, landloper, lazzarone, little monkey,
   loafer, losel, minx, monkey, mudlark, piker, puck, punk, punk kid,
   pup, ragamuffin, ragman, ragpicker, rounder, ski bum, spoiled brat,
   stiff, stray, street Arab, street arab, street urchin, sundowner,
   surf bum, swagman, swagsman, tatterdemalion, tennis bum, tramp,
   turnpiker, vag, vagabond, vagrant, waif, waifs and strays, wastrel,
   whelp, whippersnapper



The Jargon File (version 4.4.7, 29 Dec 2003):

urchin
 n.

    See munchkin.


The Free On-line Dictionary of Computing (30 December 2018):

munchkin
urchin

   /muhnch'kin/ [Squeaky-voiced little people in L. Frank Baum's
   "The Wizard of Oz"] A teenage-or-younger micro enthusiast
   hacking BASIC or something else equally constricted.  A term
   of mild derision - munchkins are annoying but some grow up
   to be hackers after passing through a larval stage.  The
   term urchin is also used.  See also wannabee, bitty box.

   [Jargon File]