Definition: Two people or entities that are alike.
Origins: In 1725, poet John Byrom coined “tweedledum” and “tweedledee” when he used those terms to make fun of two quarreling composers (Handel and Bononcini). Byrom stated that both composers had similar music and because of this, one was “tweedledum” and the other “tweedledee.” The term later gained notoriety and popularity when Lewis Carroll used the term for two twin brothers in his 1872 successful novel Through the Looking-Glass.
Ammer, Christine. The American Heritage Dictionary of Idioms. Boston: Houghton Mifflin Company, 1997, 684.