plug nickel

Some early United States coins (minted in the 18th and 19th centuries) were made with a small silver disc added to the center of the coin in the planchet (blank metal) before striking. This was done to increase the value of the metal in the coin up to the coin's face value. A plug nickel or plugged nickel is a nickel (now a five-cent coin, but originally a one-cent coin and later a three-cent coin) where the silver (center disc) has been harvested and replaced “plugged” with a different material, thus decreasing the metal value of the coin. People would often examine their change after a cash transaction to ensure they did not receive such a coin. While the reported relation to tampered coins sounds reasonable, it is not fully satisfactory. Minting of the familiar 5-cent “nickel” coin began in 1866 and continues today. These "nickels" never contained any precious metal (except for silver, briefly, during WW2). Why anyone would “plug” a nickel coin remains mysterious. (Other than the 1792 one cent pattern coin, the US mint never produced bimetallic plugged coins until the 21st century. Nickel was first used in the initial “small cent” coins from 1856 to 1864 before cents were shifted to the familiar bronze. 3 cent coins were also minted from a similar nickel alloy in 1865 to 1889).

来源：wiktionary