A more thorough explanation:

Term: In adversum
Definition: In legal terms, in adversum means against an opposing party. For example, if someone goes to court and wins a case against someone who was fighting against them, the resulting decision is called a "decree not by consent but in adversum." This means that the decision was made despite the opposition of the other party.
Example: John and Jane are in a legal dispute over a property line. John believes the line is in one place, while Jane believes it is in another. They go to court, and the judge rules in favor of John. The decision is a decree in adversum because it was made against Jane's wishes.