The phrase iron biscuit comes from a Zen koan. A teacher is described as handing a student an iron biscuit, meaning a concept that is inherently a paradox, incomprehensible in an ordinary context.
I’m just having fun with similar terms and similes. Low heartbeat humming bird. Modest Ferrari. Gregarious porcupine. Authoritative comic book. (Yes, I know that Doonesbury is the Lefty bible.) Religious science. Green thumb mechanic.
Got some good ones?