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