Literal Translation
When hens have teeth
Actual Meaning
When pigs fly
Etymology
Originally, at the end of the 18th century, this expression did not exclusively refer to teeth. To express the postponement of an action indefinitely, one could say: ‘when hens piss!’ A somewhat explicit way of telling one’s interlocutor that it will never be soon enough to check the henhouse…
In any case, it was the phrase ‘when hens grow teeth’ that prevailed over the centuries.
There many equivalents abroad. The English tend to say ‘when pigs fly’, the Spanish say ‘when frogs grow hair’ and the Germans say ‘when fish learn to fly’.