Clerks of the basilica or palace. When the Kings of France inhabited the “Palace of Justice,” the judges, advocates, proctors, and lawyers went by the common name of the clercs de la basoche; subsequently (in 1303) divided into “Clerks of the Palace,” and “Clerks of the Châtelet.” The chief of the basochians was called Le roi de la basoche, and had his court, coin, and grand officers. He reviewed his “subjects” every year, and administered justice twice a week. Henri III. suppressed the title of the chief, and transferred all his functions and privileges to the Chancellor.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894