Phrase(s): You’re telling me!
Inf. I know all too well the truth of what you are saying. • Tom: Man, it’s hot today! Bob: You’re telling me! • Jane: This food is really terrible. Sally: Wow! You’re telling me!
McGrawhill's American Idioms And Phrasal Verbs
I'm well aware of that, as in She's a terrific dancer.? You're telling me! I taught her how, or You're telling me, the prices are sky-high here. [Early 1900s]
American Heritage Idioms