“Pot calling the kettle black” – Idiom

Karen : Can you believe how much weight Lisa has gained?
Carrie : That’s not a nice thing to say about her.
Karen : It’s true. She’s always eating.
Carrie : Well, that’s the pot calling the kettle black.
Karen : What’s that supposed to mean?
Carrie : Let me just put it this way; you’re not exactly thin yourself.
Karen : It’s my clothes. They must have shrunk in the dryer.
Carrie : If you say so.

Pot calling the kettle black 
Meaning: Accusing someone of being something you are yourself; who are you to judge?
Synonymous phrase: Look who’s talking.

Here’s a poem in an early-twentieth-century school textbook runs:

“Oho!” said the pot to the kettle;
“You are dirty and ugly and black!
Sure no one would think you were metal,
Except when you’re given a crack.”

“Not so! not so!” kettle said to the pot;
“‘Tis your own dirty image you see;
For I am so clean – without blemish or blot –
That your blackness is mirrored in me.”

—Maxwell’s Elementary Grammar, 1904

Explanation:
Back in those days, pots and kettles were made of cast iron which comes in black. (These days you may find colored cast iron cookware.) So when the pot called the kettle black, the kettle told the pot, “I’m not black; it is your own reflection you see in me.”