Popularly supposed to have originated in reference to Brass Crosby, Lord Mayor of London in the 1770s, who was bold enough to defy the British parliament. However, this origin theory is dismissed by some as bogus.
bold as brass
- (simile) Very bold; very forward or impudent.
1922, James Joyce, chapter 13, in Ulysses:
The twins were now playing in the most approved brotherly fashion till at last Master Jacky who was really as bold as brass there was no getting behind that deliberately kicked the ball as hard as ever he could down towards the seaweedy rocks.