snowball's chance in hell
Definition from Wiktionary, the free dictionary
- (idiomatic, slang) Little or no likelihood of occurrence or success.
- That small boat has a snowball’s chance in hell of surviving the hurricane.
Usage notes 
Often, the expression is used in the negative for even more emphasis (implying that the referent has even less chance of success than a snowball in hell).
- That small boat doesn’t have a snowball’s chance in hell of surviving the hurricane.