A commonly used American phrase, but one that still baffles me if I stop and think about it. Why does "tell me about" actually mean, "I understand what you're talking about as I have experienced it myself". Not only are you not literally inviting the person to go into more detail, but (most confusingly) you're actually kind of suggesting that they don't need to tell you any more. Which is the very opposite of what you've said.
Employee 1: (reading a letter from management) "It says I'm being laid off. Can you believe that?"
Employee 2: (holding up a similar letter) "Tell me about it."
For such a simple and straightforward phrase, I'm confused as to how it morphed into meaning something else. You could argue that the person is being sarcastic when they say it: "Tell me about it -- as if I don't already know!", but it's usually said in a sympathetic tone, not a deriding one.
Where did it originate, and how did it get its unusual meaning? For non-native speakers, it can be very confusing!