When pigs fly 和 pigs might fly 为同义成语,意为「天方夜谭;绝不可能发生的事」,通常用来回应假设性或夸张的谈话,带有挖苦嘲弄意味。 When pigs fly 为美式英语,而 pigs might fly 为英式英语。
Pigs might fly 的原始版本为“Pigs fly in the air with their tails forward”,首次出现在英国辞典编纂者约翰·威索尔斯(John Withals) 1616 年出版的拉丁文-英文词典《A Shorte Dictionarie for Yonge Begynners》中:”Pigs fly in the ayre with their tayles forward”。

例句:

A: If we could just get the ruling party and the opposition to agree on a tax reform bill, we could bring the deficit down in no time. (A:如果我们能让执政党和反对党就税改法案达成协议,我们可以立即降低赤字)
B: Yeah, and when pigs fly! (B:是啊,但这是天方夜谭!)
A: You might get into the basketball team if you practice hard. (A:如果你努力练习,你可能进入篮球队)
B: Yes, and pigs might fly! (B:是啊,但那是绝不可能发生的事!)
A: There’s a good chance (that) she’ll have her essay finished by tomorrow, of course. (A:当然,她很可能在明天之前写完她的文章)
B: And pigs might fly. (B:天方夜谭)
A: Maybe one day we’ll be rich. (A:或许有一天我们会很有钱)
B: Oh look, I just saw a pig fly by my window. (B:哦瞧,我刚看到一只猪飞过我的窗户)