if pigs could fly

If something impossible could actually happen. (Used to show skepticism or cynicism over some hypothetical situation or outcome.) A: "If we could just get Democrats and Republicans to agree on a tax reform bill, we could bring the deficit down in no time." B: "Sure, and if pigs could fly, we'd have a whole new mode of transportation at our fingertips! Get real." I'm sure that David would be happy to pay for everyone—if pigs could fly. Billy as class valedictorian? Yeah, maybe if pigs could fly!