What is Petty Bag Office?

Legal Definition
In English law. An office in the court of chancery, for suits against attorneys and officers of the court, and for process and proceedings by extent on statutes, recognizances, ad quod damnum, and the like. Termes de la Ley.
-- Black's Law Dictionary
Legal Definition
An office in the English chancery court where writs were issued for and against court officers. See 6 Johns. (N. Y.)367.
-- Ballentine's Law Dictionary