Matt Bateman recently quipped that “the pot is correct when it calls the kettle black,” and someone replied that it’s not:
The saying originates from a time when kettles were polished metal, and pots were cast iron. The pot, seeing its own reflection in the shiny tin kettle, calls the kettle black.
Naturally, I had to look this up:
The earliest appearance of the idiom is in Thomas Shelton’s 1620 translation of the Spanish novel Don Quixote. The protagonist is growing increasingly restive under the criticisms of his servant Sancho Panza, one of which is that “You are like what is said that the frying-pan said to the kettle, ‘Avant, black-browes’.”
[…]
It is identified as a proverb (refrán) in the text, functioning as a retort to the person who criticises another of the same defect that he plainly has.
[…]
An alternative modern interpretation,[8] far removed from the original intention, argues that while the pot is sooty (from being placed on a fire), the kettle is polished and shiny; hence, when the pot accuses the kettle of being black, it is the pot’s own sooty reflection that it sees: the pot accuses the kettle of a fault that only the pot has, rather than one that they share.