The notion of a criticism a person is making of another could equally well apply to themself
That means I am accusing you, and/or judging you negatively about it, of which I am guilty myself.
For example, I could say something nasty about you getting a speeding ticket, but then you find that my traffic record shows multiple offenses too. It's hypocrisy.
It comes from the fact that both cooking vessels were kept hot in the fire. The bottoms of both would be black with soot. One is not any cleaner than the other.