The word brazen comes from Old English bræsen ,  meaning "made of brass", linking directly to the word brass (from Pro-Germanic brasno).  Its figurative sense of being bold, impudent, or shameless developed from the idea of a face that is hard like brass, unable to show shame, with the "brazen-faced" usage appearing in the 1570s.  

According to the OED, the adjective "brazen" (meaning "of brass") appears in Old English (before 1150), while the figurative senses ("hardened," "impudent") emerge later, with "brazen-faced" noted from the 1570s, and the verb "to brazen" (to act impudently) first appearing around the mid-1500s, with early use by Hugh Latimer before 1555.  The figurative shift of the verb "to brazen it" (to face something impudently) emerged, suggesting a hard, unmoving, brass-like demeanor, was first used in the writings of G. Harvey in 1573.

Examples of "brazen" used in a sentence: 
"It was a brazen attempt to rig the result."
"Is he going to be this brazen, this bold all the time?"
"He exhibited a brazen disregard from other people's feelings." 
"The brazen sounds of the city's brass bands were muted."
"There was the brazen shooting of 79-year-old Jose Quirin."
"In fact, many birds are quite skilled at bold and brazen theft."