(as) rare as rocking horse crap

vulgar slang Extremely rare or unlikely; all but, or most likely, impossible or non-existent. Primarily heard in Ireland. An honest person in politics? Yeah, that's as rare as rocking horse crap. I'm sorry to say, but the part this engine needs is rare as rocking horse crap. They haven't manufactured these in nearly 50 years! Passionate love like you find in a poem has always seemed rare as rocking horse crap in my experience.