The word-origin seems to depend on geography. The OED notes that "In Scotland the usual name for a baker's plain bun; in U.S. usually a small flat sweet cake (a biscuit in U.K.)." In the US, "Cookie" is from the Dutch koekje, the diminutive of koek - cake; the origin of the Scottish term remains obscure.

The first mention of the Scottish term is from 1754, while the US usage appears in the early 19th century. The use in slang (smart cookie, etc.) is first attested in the 1920s.