A small rock intended as a humorous novelty alternative to keeping an animal or growing plants; (hence) something seen as little more than a passing fad, or as pointless or useless.
Origin
1970s; earliest use found in Newsweek.
Definition of pet rock in US English:
pet rock
noun
A small rock intended as a humorous novelty alternative to keeping an animal or growing plants; (hence) something seen as little more than a passing fad, or as pointless or useless.