Meaning:

Never.

If you you say when pigs fly, you mean something will never happen.

People sometimes say, "And pigs might fly" to mean something a person just said will never happen or is impossible.

Example:

A: Do you think she'll ever come back and work with us again?
B: When pigs fly she will. (She will never come back again.)

A: We might get a raise when another person becomes the president (of this company).
B: And pigs might fly. (It'll never happen.)

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