As far as i know it's about something impossible that happened, for instance: someone says something that's actually impossible so the other person could reply like: Yeah, if pigs could fly, or: yeah, if it would snow in hell. so the impossible became a fact thus making it snow in hell.
As far as i know it's about something impossible that happened, for instance: someone says something that's actually impossible so the other person could reply like: Yeah, if pigs could fly, or: yeah, if it would snow in hell. so the impossible became a fact thus making it snow in hell.