"Be one's case" can be defined as demonstrating or providing evidence to support one's argument. The antonyms for this phrase can be "oppose", "dispute", "contradict", or "reject". Opposing someone's case implies disagreeing with their point of view or argument. When one disputes someone's case, they demonstrate that the evidence or reasons mentioned are not valid. Similarly, when someone contradicts someone's case, they provide counter-evidence or arguments that go against their point of view. Lastly, rejecting someone's case indicates denying their arguments or evidence without providing counterevidence. All these antonyms show how the phrase "be one's case" can be challenged or discredited in a debate or argument.