don’t take any wooden nickels

Protect yourself (against fraud, loss, and so on). This warning against counterfeit coins dates from about 1900 and is distinctly American in origin, the nickel being a U.S. or Canadian five-cent coin. Why a wooden coin was selected is not known. Presumably making coins of wood would always have been more expensive than the intrinsic value of metal coins. Several writers suggest it replaced don’t take any wooden nutmegs, a now obsolete saying dating from colonial times when sharp traders sold wooden nutmegs mixed in with the real spice. In print the expression is found in Ring Lardner’s story, The Real Dope (1919), “In the mean wile—until we meet again—don’t take no wood nickles [sic] and don’t get impatient and be a good girlie.”