We use this expression to refer to someone who criticises someone else, for something they they themselves are guilty of.
Example
“You’re greedy.” “Pot calling the kettle black?”
Origins
First used in the literature of the 1600s – notably Don Quixote by Cervantes – this expression has its origins in the Medieval kitchen, when both pots and kettles were made from sturdy cast iron and both would get black with soot from the open fire.