Definition
This idiom is used to retort when someone criticizes you for a fault or behavior that they themselves exhibit.
你也不一樣！
"You're always late!" "Oh, look who's talking!"
「你總是遲到！」「哦，看看是誰在說話！」
He called me a big spender? Look who's talking!
他叫我大花錢？看看是誰在說話！
"You should exercise more," she said, look who's talking, she never goes to the gym.
「你應該多運動，」她說，看看是誰在說話，她從不去健身房。