The term “good” very much describes something real (and natural), otherwise we wouldn’t be able to think of it.
That doesn’t seem like a consistently valid rule.
As counter-examples, here are some words that we have thought of, and that we can use consistently and appropriately in context, but that do not describe real or natural things:
Unicorn
Witch
Luminiferous aether
Bottomless pit
Immovable object
Irresistible force
Philosopher’s stone
Faster-than-light communication
Ideal gas
Frictionless surface
Spherical cow
Halting oracle
“Yo mama’s so fat she has different area codes for the phones in her left and right pockets”
That doesn’t seem like a consistently valid rule.
As counter-examples, here are some words that we have thought of, and that we can use consistently and appropriately in context, but that do not describe real or natural things:
Unicorn
Witch
Luminiferous aether
Bottomless pit
Immovable object
Irresistible force
Philosopher’s stone
Faster-than-light communication
Ideal gas
Frictionless surface
Spherical cow
Halting oracle
“Yo mama’s so fat she has different area codes for the phones in her left and right pockets”