To issue a judgment that goes against someone's or something's case. The judge ruled against the father, stating that he had not shown adequate reason to have sole custody of the children.
rule against someone or something
to give a judgment against someone or something. The judge ruled against the prosecutor. The judge ruled against my motion.
rule against
v.
To deliver a judgment that is not favorable to someone or something: The court ruled against the plaintiff, and no damages were awarded.