Old English bræs "brass, bronze," originally in reference to an alloy of copper and tin (now bronze), later and in modern use an alloy of two parts copper, one part zinc. A mystery word, with no known cognates beyond English. Perhaps akin to French brasser "to brew," because it is an alloy. It also has been compared to Old Swedish brasa "fire," but no sure connection can be made. Yet another theory connects it with Latin ferrum "iron," itself of obscure origin.As brass was unknown in antiquity, use of the word in Bible translations, etc., likely means "bronze." The Romans were the first to deliberately make it. Words for "brass" in other languages (such as German Messing, Old English mæsling, French laiton, Italian ottone) also tend to be difficult to explain.The meaning "effrontery, impudence" is from 1620s. Slang sense of "high officials" is first recorded 1899. The brass tacks that you get down to (1897) probably are the ones used to measure cloth on the counter of a dry goods store, suggesting precision. Slang brass balls "toughness, courage" (emphatically combining two metaphors for the same thing) attested by 1960s.