Pig’s can’t fly! It’s impossible, which is precisely what the expression means.We say it as a response when someone tells us something that we think is impossible or very unlikely. For example, if person A says “Do you think Mary will quit her job now that she’s pregnant?” and person B responds “Yeah, when pigs fly! There is no way she is giving up her career!” Person B thinks that it is very unlikely that Mary will quit her job.