Here's why:
* Meaning: "Passable" means "adequate" or "just good enough." It suggests a low bar for quality. Prefixes like "un-" or "in-" imply a negative, which doesn't fit with the meaning of "passable."
* Usage: We typically use "passable" on its own, without prefixes.
If you need a word to convey a similar meaning, here are some alternatives:
* Tolerable: Acceptable, though not particularly good.
* Mediocre: Average, neither good nor bad.
* Adequate: Sufficient, but not excellent.
* Marginal: Just barely good enough.
Let me know if you have other words you'd like to explore!