A similar phrase is "the pot calling the kettle black". You say "look who's talking!" as a retort when you know the other person should follow their own advice before giving it.

hi xue'er , it is actually an idiom which means "you're in no position to criticize." this is when you find fault someone in which you bear the same characteristics (it could be physical, mental or behavioural). for instance you're a fat guy who always criticize other fat guys, you would tell: "look at those fat morons, they can't manage to loose their weight." someone will tell you: "look who's talking!" - so "look who's talking" here suggests that to look at yourself first before you criticize others (the fact that you too is a fat guy), and not you're in no position to say that... :)

hmnnn how can i explain this :P let's see when you suggest or say something that you actually don't agree or do. For example: A: Yea John, you should do your homework everyday is good for you B: Look who's talking! A: hehe i know i dont do it ... =)

Simply put: Its something which you say when somebody blames you for something which they do themselves!