Bad is defined as the opposite of good, so this means something assessed as such is unfit or morally reprehensible.
Fitness
Fitness implies appropriateness, that something does what it should without waste or failure. It says that the thing or person is capable, that the job is done well. When something is fit for purpose we can things like 'that is a good joke' or 'she is a good doctor'.
Morality
When something is moral, we say an act is good or bad. We also may say that the person is good or bad, although this implies that a person who does one bad thing has permanently bad intent and will always do bad things, and vice versa.
This makes good and bad uniquely human. Animals do not have morals, so predatory and possibly cruel (in our view) acts are neither good nor bad. We cannot say, for example, that a lion which kills a terrified buffalo is bad (although from a fitness viewpoint, the act is bad for the buffalo and good for the lion).