The term "being one's case" refers to the process of making a case or argument for oneself. When someone is being one's case, they are presenting evidence or arguments to support their position on a particular issue. The antonym for "being one's case" is simply "not being one's case" or "not making a case." In this case, a person is not presenting any evidence or arguments to support their stance on a specific issue. It can mean that they have chosen to remain silent or have not yet prepared a well thought-out argument. It can also indicate that they do not wish to engage in a particular discussion or debate.
