rocking horse shit

vulgar slang Something is extremely rare or unlikely; all but, or most likely, impossible or non-existent. Often used in the phrase "(as) rare as rocking horse shit." Primarily heard in Ireland. An honest person in politics? Yeah, that's as rare as rocking horse shit. I'm sorry to say, but the part this engine needs is rare as rocking horse shit. They haven't manufactured these in nearly 50 years!