The phrase "bold as brass" is typically used to refer to a person acting without shame or fear of consequences. Such a person would feel no embarrassment in situations where others would be mortified, carrying on with a confidence that borders on arrogance or impudence. This alliterative phrase dates to at least 18th century London, where it was first recorded in print, but "bold as brass" might go back even further as an oral idiom.
Frequently, the word “brass” has been used to refer to more than the metal, often used to describe shamelessness, among other definitions. Like the metal, brass as a trait is bright and flashy, likely to draw attention, but there are other less-flattering parallels as well. Brass in a person might be perceived as cheap — a poor imitation of gold’s virtue. Calling a person "bold as brass" is not a compliment.
The use of "brass" in this context dates to at least the 1500s, with the first known instance in print attributed to William Shakespeare. By the 1700s, "brass" as shamelessness was in common usage. The phrase "bold as brass," however, is not known to have been used in print until 1789, in Life’s Painter of Variegated Characters in Public and Private Life, with Political Strokes on the Ticklish Times by George Parker.
Many people, however, believe the phrase to have been born 19 years earlier, in 1770, inspired by Brass Crosby, Chief Magistrate of London. The London Evening Post printed a report detailing debates held in Parliament, the reporting of which was not permitted at the time. As a result, two printers were arrested and taken to appear before the magistrate. Crosby defied Parliament and released the men, and when a messenger was sent with an order for Crosby to arrest another printer, Crosby instead arrested the messenger.
Crosby himself was eventually jailed, although he received a surge of popular support for his actions. No news report or written account of the day is known to have referred to him as "bold as brass," but many experts believe that this incident is responsible for the birth of the phrase. It does seem likely that when Parker wrote his book, he was aware of Brass Crosby’s reputation, but it is possible that his inspiration ran no deeper than a bit of alliteration and some clever wordplay. Whatever the source, the scandalous and the shameless have been declared to be "bold as brass" ever since.