Pinchbeck is a specific type of brass, an alloy of copper and zinc, formulated to resemble gold as closely as possible as a much cheaper alternative. Two common formulations of pinchbeck were 89% copper and 11% zinc, or 93% copper and 7% zinc.
Pinchbeck was named after its original inventor, the London clockmaker Christopher Pinchbeck (1670-1732).
In the 18th and 19th centuries, pinchbeck was used to mass produce large quantities of cheap jewelry, and eventually the word "pinchbeck" came to be used as a short hand for cheap, imitation jewelry in particular, and anything fake or fraudulent in general.