Learn English Online - Idiomatic Expressions
Definition of Idiomatic Expressions

What does when pigs fly mean?
Meaning of idioms with examples...
when pigs fly
If you say when pigs fly you mean that something will never happen.This phrase is used presumably due to the unlikelihood that pigs will ever evolve wings.
Example(s):
Sure she'll help us clean the house — when pigs fly.
This idiom is in the animals category