(idiomatic) A time that never occurs; never; when pigs fly.
1923, [François Béroalde de Verville], “Origin of the Decretals”, in Arthur Machen, transl., Fantastic Tales or The Way to Attain—a Book Full of Pantagruelism Now Done for the First Time in English, Carbonnek [i.e., London]: Privately printed, →OCLC, page 98: