Unhealthy means bad for the body, or not in good health. A diet of chips, or a person who looks grey.
It works in both directions, describing the cause and the state, so unhealthy food makes for an unhealthy heart. It also describes appearance, and an unhealthy pallor is one of the standard phrases of fiction. The most interesting use is figurative and about the mind, since an unhealthy interest or an unhealthy obsession means something has gone wrong in the proportion of it. That sense is always a warning.
From un- + healthy.
Etymology from Wiktionary, CC BY-SA