إما عامية وهي المبتذلة لظهور الجامع فيهما
A metaphor may be general and commonplace because its shared feature is apparent in both terms.
ʿĀmmiyyah indicates broad intelligibility; mubtadhalah means conventional or frequently used here. It is a technical description, not a moral judgment that the metaphor is improper.
The expression still transfers meaning figuratively, but the likeness is sufficiently recognizable that an ordinary competent hearer can infer its intended point without extended reflection.