I think the original is 除非太阳打西边出来 meaning "when the sun rises in the west". I think it's used to refer to something that is definitetly not going to happen, in English it would be "when pigs fly"

let me have a try..Literallly it means "the sun rises from the west", but it evidently breaks the rules of nature, so we can imply that this metaphorical chinese slang is to indicate something that is unlikely or rarely to happen indeed happens. When using this slang, the speaker is usually surprised to the incidents.

I think the original is 除非太阳打西边出来 meaning "when the sun rises in the west". I think it's used to refer to something that is definitetly not going to happen, in English it would be "when pigs fly"