Tell me about it!

[Nightmare85]

Hello,
I heard/read that "Tell me about it!" is an idiom which is used when you already know something.
Did you know the earth is circular?
"Tell me about it!"

However, if you are very interested in something and don't know anything about it, what would you say?
I would have said "Tell me about it!" because I did not know it's an idiom...

Of course there are plenty of versions:
Explain this more detailed to me.
Sounds interesting, tell me more.
However, would
"Tell me about it!"
always mean that you already know it?

Cheers!

[euncu]

Person 1: My boss really hasn't got a slightest idea how to run a company .
Person 2: Tell me about it! My boss is as incompetent as yours.


or

Person 1:I had a terrible day!
Person 2:Oh,Tell me about it! (Person 2 doesn't want Person 1 to tell about his day. He/she just means that he/she had a bad day too.)

[fighting spirit]

"Tell me about it!" is an idiom which is used when you already know something.
However, if you are very interested in something and don't know anything about it, what would you say?
I would have said "Tell me about it!" because I did not know it's an idiom...

You can distinguish an idiomatic use from ordinary with the intonation. You won't pronounce it the same way when you want to say that you already know something.

[Linguist__]

(Not a teacher)

And euncu's example could be non-idiomatic too:

Person 1:I had a terrible day!
Person 2:Oh,Tell me about it.

Person 2 might really be requesting Person 1 to tell him/her about the horrible day.

As fighting spirit said, if context doesn't give it away, intonation/tone will. Normally, 'Tell me about it' as an idiom is followed by the person's description of the event, and so will have an intonation that suggests the speaker isn't finished.