Plutarch, Life of Crassus
18.2 (tr. Rex Warner):
Crassus boastfully replied that he would give them his answer in Seleucia, and the eldest of the ambassadors, Vagises, burst out laughing and, pointing to the palm of his upturned hand, said: 'Hair will grow here, Crassus, before you set eyes on Seleucia.'
This is an example of the rhetorical trope known as adynaton
(impossibility). Another example is Herodotus 5.92.a.1 (tr. George Rawlinson):
Surely the heaven will soon be below, and the earth above, and men will henceforth live in the sea, and fish take their place upon the dry land, since you, Lacedaemonians, propose to put down free governments in the cities of Greece, and to set up tyrannies in their room.
In English, we say, "When pigs fly..."