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]