unjust, unrighteous: failing to do what is right, the opposite of the righteous person
This word is built by adding a negating prefix to the adjective for "just" or "righteous" (δίκαιος), so at root it means simply "not righteous, not just." It describes a person, and occasionally a thing, that fails to do what is right, whether toward God or toward other people.
Most often it labels a whole class of people. Set against "the righteous," it names those who are out of a right relationship with God, the unrighteous who are raised or judged alongside the righteous. It is the matching adjective to the noun for the wrongdoing itself (ἀδικία): that word names the injustice, while this word names the person who is unjust.
In a narrower use, especially where money is in view, the word shades toward "dishonest" or "untrustworthy." When Jesus sets being "faithful" against being unrighteous in the handling of wealth, the concern is not fairness in the abstract but whether a person can be trusted with what belongs to another.