The Collaborative International Dictionary of English v.0.48:

Shrub \Shrub\, n. [Ar. shirb, shurb, a drink, beverage, fr.
   shariba to drink. Cf. Sirup, Sherbet.]
   A liquor composed of vegetable acid, especially lemon juice,
   and sugar, with spirit to preserve it.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Shrub \Shrub\, n. [OE. schrob, AS. scrob, scrobb; akin to Norw.
   skrubba the dwarf cornel tree.] (Bot.)
   A woody plant of less size than a tree, and usually with
   several stems from the same root.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Shrub \Shrub\, v. t.
   To lop; to prune. [Obs.] --Anderson (1573).
   [1913 Webster]

WordNet (r) 3.0 (2006):

shrub
    n 1: a low woody perennial plant usually having several major
         stems [syn: shrub, bush]

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "shrub":
   Persian berry, alder, azalea, bayberry, blackberry, blackthorn,
   box, bramble, brier, brier bush, broom, bush, caper, cinchona,
   coca, coffee, cranberry, currant, daphne, elder, forsythia,
   frangipani, fuchsia, furze, gardenia, gooseberry, gorse,
   greasewood, guava, haw, heather, hemp tree, henna, hibiscus, holly,
   huckleberry, hydrangea, indigo, juniper, jute, laurel, leatherleaf,
   lilac, magnolia, manzanita, mock orange, mountain lilac, myrtle,
   ninebark, oleander, photinia, poison sumac, privet, pussy willow,
   rhododendron, rose of Sharon, rosebay, rosemary, sage, sagebrush,
   sand myrtle, scrub, shrubbery, sisal, snow wreath, snowberry,
   spiraea, sumac, symplocos, syringa, tamarisk, veronica,
   witch hazel, yellowroot