Satanic things are very bad or evil. You might describe your math teacher's pop quizzes as satanic, but that would be a bit of an exaggeration.
The very first recorded use of the adjective satanic was in John Milton's 1667 epic poem "Paradise Lost," where it literally described things having to do with Satan, or the devil. Some people continue to use satanic to describe things they associate with hell or the devil, while many others use it as a synonym for "cruel" or "evil." The Hebrew root, satan, means "to oppose or plot against."