Variants of Nim have been played since ancient times. The game is said to have originated in China—it closely resembles the Chinese game of "Tsyan-shizi", or "picking stones"—but the origin is uncertain; the earliest European references to Nim are from the beginning of the 16th century. Its current name was coined by Charles L. Bouton of Harvard University, who also developed the complete theory of the game in 1901, but the origins of the name were never fully explained. The name is probably derived from German nimm meaning "take [imperative]", or the obsolete English verb nim of the same meaning.