The Collaborative International Dictionary of English v.0.48:

Snowball \Snow"ball`\, v. t. [imp. & p. p. Snowballed; p. pr.
   & vb. n. Snowballing.]
   To pelt with snowballs; to throw snowballs at.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Snowball \Snow"ball`\, v. i.
   1. To throw snowballs.
      [1913 Webster]

   2. To increase in magnitude at an accelerating rate,
      achieving large proportions; -- by analogy with a snowball
      rolling down a steep hill, causing a large snow slide.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Snowball \Snow"ball`\, n.
   1. A round mass of snow pressed or roller together, or
      anything resembling such a mass.
      [1913 Webster]

   2. (Bot.) The Guelder-rose.
      [1913 Webster]

   3. (Bot.) a shrub of the genus Viburnum, having large
      clusters of white flowers.
      [PJC]

   Snowball bush or Snowball tree (Bot.), the
      Guelder-rose.

   a snowball's chance in hell, [Colloq.] no chance; an
      infinitesimal chance.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

snowball
    n 1: plant having heads of fragrant white trumpet-shaped
         flowers; grows in sandy arid regions [syn: snowball,
         sweet sand verbena, Abronia elliptica]
    2: ball of ice cream covered with coconut and usually chocolate
       sauce
    3: ball of crushed ice with fruit syrup
    4: snow pressed into a ball for throwing (playfully)
    v 1: increase or accumulate at a rapidly accelerating rate
    2: throw snowballs at

Moby Thesaurus II by Grady Ward, 1.0:

110 Moby Thesaurus words for "snowball":
   accrue, accumulate, accumulation, acervation, advance, agglomerate,
   agglomeration, aggregate, aggregation, amassment, appreciate,
   augment, avalanche, ball, balloon, bead, blizzard, bloat, boom,
   breed, broaden, build, bulk, chunk, congeries, conglobation,
   conglobulate, conglomerate, conglomeration, crescendo, crystal,
   cumulation, develop, dilate, distend, driven snow, enlarge, expand,
   extend, fill out, flake, flurry, gain, gain strength, gathering,
   get ahead, globe, glomeration, go up, gob, granular snow, greaten,
   grow, hunk, igloo, increase, intensify, lump, mantle of snow, mass,
   mogul, mount, multiply, mushroom, proliferate, puff up, rise,
   run up, shoot up, slosh, slush, snow, snow banner, snow bed,
   snow blanket, snow blast, snow fence, snow flurry, snow roller,
   snow slush, snow squall, snow wreath, snow-crystal, snowbank,
   snowbridge, snowcap, snowdrift, snowfall, snowfield, snowflake,
   snowland, snowman, snowscape, snowshed, snowslide, snowslip,
   snowstorm, sphere, spherify, spread, stockpile, strengthen,
   stretch, swell, tumefy, upsurge, wad, wax, wet snow, widen