Definition of snowball noun from the Oxford Advanced American Dictionary

snowball

noun
/ˈsnoʊbɔl/
 
 
1 [countable] a ball that you make out of snow to throw at someone or something in a gamea snowball fightChildren were throwing snowballs at the passing traffic.2 [singular] (often used as an adjective) a situation that develops more and more quickly as it continuesAll this publicity has had a snowball effect on the sales of their latest album.3 [countable] a cocktail (= a type of alcoholic drink) that contains advocaat and lemonadeIDIOM

not have a snowball's chance in hell

(informal) to have no chance at allnot have a snowball's chance in hell