E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
At the bar. As the prisoner at the bar, the prisoner in the dock before the judge.
Trial at bar, i.e. by the full court of judges. The bar means the place set apart for the business of the court.
To be called to the bar. To be admitted a barrister. The bar is the partition separating the seats of the benchers from the rest of the hall. Students having attained a certain status used to be called from the body of the hall within the bar, to take part in the proceedings of the court. To disbar is to discard from the bar. Now, to be called within the bar means to be appointed kings (or queens) counsel; and to disbar means to expel a barrister from his profession.