E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Such a number of persons as are necessary to make up a committee or board; or certain justices without the presence of whom the rest cannot act. Thus, suppose the commission to be named A, B, C, D, E, etc., it would runOf these I wish [A, B, C, D, or E] to be one (quorum unum esse volumus). These honoured names are called Justices of the Quorum. Slender calls Justice Shallow justice of the peace and quorum. (Shakespeare: Merry Wives of Windsor, i. 1.)