Traditionally taken as a type of hardness or insensitivity; impudence, effrontery, nerve.
Formerly (from late Middle English to the late 18th century), brass was used for copper or bronze coin; from the late 16th century, it has been used informally to mean ‘cash’.
In the UK, brass also denotes a memorial, typically a medieval one, consisting of a flat piece of inscribed brass, laid in the floor or set into the wall of a church.
The word is recorded from Old English (in form bræs), but is of unknown origin.
brass hat an army officer of high rank (having gold braid on the cap); the term may be used pejoratively, to indicate someone seen as out of touch with the fighting forces.
the brass ring in North America, an informal expression for success, typically regarded as a reward for ambition or hard work, originally with reference to the reward of a free ride given on a merry-go-round to the person hooking a brass ring suspended over the horses.
cold enough to freeze the balls off a brass monkey means bitterly cold; the phrase is often said to come (in the late 19th century) from a type of brass rack or ‘monkey’ in which cannonballs were stored and which contracted in very cold weather, ejecting the balls. However, the term ‘monkey’ is not otherwise recorded in this sense, and the rate of contraction of brass in cold temperatures is unlikely to be sufficient to cause the reputed effect. The phrase is also first recorded as ‘freeze the tail off a brass monkey’. It therefore seems most likely that the phrase is simply a ribald allusion to the fact that metal figures will become very cold to the touch in cold weather (and some materials will become brittle).
get down to brass tacks come to the essential details, reach the real matter in hand; the term, which is originally US, is recorded from the late 19th century.