The Collaborative International Dictionary of English v.0.48:

Scrub \Scrub\ (skr[u^]b), n.
   1. One who labors hard and lives meanly; a mean fellow. "A
      sorry scrub." --Bunyan.
      [1913 Webster]

            We should go there in as proper a manner as
            possible; nor altogether like the scrubs about us.
                                                  --Goldsmith.
      [1913 Webster]

   2. Something small and mean.
      [1913 Webster]

   3. A worn-out brush. --Ainsworth.
      [1913 Webster]

   4. A thicket or jungle, often specified by the name of the
      prevailing plant; as, oak scrub, palmetto scrub, etc.
      [1913 Webster]

   5. (Stock Breeding) One of the common live stock of a region
      of no particular breed or not of pure breed, esp. when
      inferior in size, etc. [U.S.]
      [1913 Webster]

   6. Vegetation of inferior quality, though sometimes thick and
      impenetrable, growing in poor soil or in sand; also,
      brush; -- called also scrub brush. See Brush, above.
      [Australia & South Africa]
      [Webster 1913 Suppl. +PJC]

   7. (Forestry) A low, straggling tree of inferior quality.
      [Webster 1913 Suppl.]

   Scrub bird (Zool.), an Australian passerine bird of the
      family Atrichornithidae, as Atrichia clamosa; --
      called also brush bird.

   Scrub oak (Bot.), the popular name of several dwarfish
      species of oak. The scrub oak of New England and the
      Middle States is Quercus ilicifolia, a scraggy shrub;
      that of the Southern States is a small tree (Quercus
      Catesbaei); that of the Rocky Mountain region is Quercus
      undulata, var. Gambelii.

   Scrub robin (Zool.), an Australian singing bird of the
      genus Drymodes.
      [1913 Webster]

WordNet (r) 3.0 (2006):

scrub bird
    n 1: small fast-running Australian bird resembling a wren and
         frequenting brush or scrub [syn: scrubbird, scrub-bird,
         scrub bird]