" When pigs fly" is meant to indicate an impossible condition.

“When pigs fly” 是指不可能的情况。

A spokesman for the Palestinian president quickly responded to the Israeli leader’s speech to Congress, saying Netanyahu had put more obstacles in the way of a Middle East peace agreement by imposing impossible conditions.

巴勒斯坦总统的发言人迅速回应了以色列领导人在国会的讲话，称内塔尼亚胡强加了不可能的条件，为达成中东和平协议设置了更多障碍。