things that do not exist; horns on rabbits, fur on turtles
A Buddhist-derived yojijukugo used idiomatically to refer to something that is impossible or nonexistent, like a rabbit's horns or a turtle's fur. Rare in everyday speech; mostly encountered in literary or religious contexts.
兎角亀毛 is used as a metaphor for things that do not exist in this world.
From the Buddhist expression 兎角亀毛, literally “a rabbit’s horns and a turtle’s hair,” conventional examples of things that cannot exist.