“Inimitable” comes from the Latin “imitari,” meaning “to imitate,” and the prefix “in-,” meaning “not” — English borrowed the word for “unable to be imitated” from French in the 15th century. Since its inception in 1789, the U.S. Treasury Department has endeavored to make its currency as inimitable as possible. To ward off counterfeiters, the Treasury has instituted numerous measures to change the appearance of its bills, including special watermarks, security strips, color-shifting ink, and ongoing tweaks to the paper composition.