An iniquity is a really unfair or really immoral act. If you're a journalist, you may uncover a terrible iniquity that forces a famous politician to step down.
Iniquity comes from Latin, combining the prefix in-, which means “not,” and aequus, which means “equal” or “just.” So iniquity literally means “not just.” Iniquity can also be used to say that something lacks moral or spiritual principles. Some would call Las Vegas a “den of iniquity,” implying that sinful or wicked behavior occurs there. Others would just call it a vacation.