The oldest reference to be found is from Juvenal from the late First Century of the Christian Era – rara avis in terris nigroque simillima cycno. With apologies to the women reading this, Juvenal railed that finding a perfect wife is akin to finding a black swan (a rare bird). Shakespeare may well have used the term too – but by his day in the sixteenth and seventeenth centuries the black swan was used to describe something that was impossible. Although the expression is not found in his works (perhaps he preferred to invent his own) the black swan represented that which was impossible (or nearly so) and therefore simply could not be.
