Pot Calling The Kettle Black
idiomatic phrase that people sometimes use to point out hypocrisy. It means that someone is criticizing another person for a fault they have even though the criticizer is also guilty of doing the same thing.
When a cast-iron pot (or kettle) is held over a fire, the bottom eventually darkens. Why? Because the flame the pot is being held over causes soot, a black powdery substance, to accumulate under the pot. With enough time, the bottom of the pot can turn black.