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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 (e.g. 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.
which is an alloy of copper and zinc, was not known till the thirteenth century. What is designated by this word in Scripture is properly copper (Deut. 8:9). It was used for fetters (Judg. 16:21; 2 Kings 25:7), for pieces of armour (1 Sam. 17:5, 6), for musical instruments (1 Chr. 15:19; 1 Cor. 13:1), and for money (Matt. 10:9). It is a symbol of insensibility and obstinacy in sin (Isa. 48:4; Jer. 6:28; Ezek. 22:18), and of strength (Ps. 107:16; Micah 4:13). The Macedonian empire is described as a kingdom of brass (Dan. 2:39). The "mountains of brass" Zechariah (6:1) speaks of have been supposed to represent the immutable decrees of God. The serpent of brass was made by Moses at the command of God (Num. 21:4-9), and elevated on a pole, so that it might be seen by all the people when wounded by the bite of the serpents that were sent to them as a punishment for their murmurings against God and against Moses. It was afterwards carried by the Jews into Canaan, and preserved by them till the time of Hezekiah, who caused it to be at length destroyed because it began to be viewed by the people with superstitious reverence (2 Kings 18:4). (See NEHUSHTAN.) The brazen serpent is alluded to by our Lord in John 3:14, 15. (See SERPENT.)