E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
An egg without a shell. Dr. Johnsons notion that the wind egg does not contain the principle of life is no more correct than the superstition that the hen that lays it was impregnated, like the Thracian mares, by the wind. The usual cause of such eggs is that the hen is too fat.