The Pot Calling the Kettle Black

This image comes from Miguel de Cervantes's Don Quixote, by way of Thomas Shelton's 1620 English translation, where the phrase appears as Sancho Panza is compared to "the frying-pan" telling "the kettle" to get away for having a black face. In a kitchen built around open fires, both a cooking pot and a kettle would be scorched black with soot from constant use, which is exactly the joke: one sooty object mocking another sooty object for being sooty is absurd, since neither has any standing to criticise the other. The phrasing shifted over the following decades from pan-and-kettle into the pot-and-kettle version used today.