1
general::
Shameless, audacious, impudent. For example, No one had invited her to the wedding, but she showed up at the church, bold as brass. This alliterative simile plays on brass meaning "shamelessness." [c. 1700]
American Heritage Idioms
2
general::
Phrase(s): *bold as brass
very bold; bold to the point of rudeness. (*Also: as ~.) • Lisa marched into the manager’s office, bold as brass, and demanded her money back. • The tiny kitten, as bold as brass, began eating the dog’s food right under the dog’s nose.
McGrawhill's American Idioms And Phrasal Verbs