What Does the Expression “Pot Calling the Kettle Black” Mean and Where Did the Saying Come From?

The expression “The pot calling the kettle black” first entered a dictionary in 1699 with the explanation,

“When one accuses another of what he is as deep in himself.”

When kitchen stoves were fired by wood and coal, both the kettle and the pot would become black through time, so both were equally tarnished.

Another explanation is that because both were made of copper, the more prized kettle might have been polished, which would offer the grungy pot a reflection of himself.