From Old English 'bræsen' (made of brass) — the shift from material to metaphor drew on brass's hardness, loud ring, and bold golden color.
Bold and shameless; made of or resembling brass.
From Old English 'bræsen' (made of brass), an adjectival derivative of 'bræs' (brass, bronze), from Proto-Germanic *brasa- (fire, glowing coals) and cognate with Swedish 'bräsa' (to rush, blaze), Danish 'brase' (to fry, crackle), suggesting the original meaning was 'glowing metal' rather than a specific alloy. PIE root *bhreg- (to gleam, shine) may underlie the Germanic forms. The literal 'made of brass' sense dominated through
The English slang 'brass neck' (meaning impudence or nerve) and 'brassy' (bold and showy) share the same metal metaphor as 'brazen.' In several British dialects, 'brass' means money — brass coins being the lowest denomination — and 'brassed off' means annoyed. The metal permeates English idiom.