pigs might (or can) fly used ironically to express disbelief. chiefly British

☞ Pigs fly in the air with their tails forward was a proverbial saying in the 17th century; the current version dates back to the late 19th century, and the first recorded use is by Lewis Carroll.

1973Jack HigginsA Prayer for the Dying 'Something could come out of that line of enquiry.' 'I know…Pigs might also fly.'