What is "a snowball's chance in hell"

Snowball \Snow"ball`\, n.

1. A round mass of snow pressed or roller together, or anything resembling such a mass.

2. (Bot.) The Guelder-rose.

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

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

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