You must have mistaken me for someone who gives a shit

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you (must) have mistaken me for someone who gives a shit

rude I don't care in the slightest about what you are saying. A: "Please don't discuss that topic on the air. You might offend some of our listeners." B: "Oh, I'm sorry, you must have mistaken me for someone who gives a shit." A: "I'm sorry, boss, but I don't know if I'll have the project done on time. I'm just dealing with a lot of personal issues." B: "You've clearly mistaken me for someone who gives a shit. Get it done by Friday, or you can look for a new job."
See also: give, have, mistaken, shit, someone, who
Farlex Dictionary of Idioms.

You must have mistaken me for someone who gives a shit

and YMHMMFSWGAS
sent. & comp. abb. What makes you think I care about what you are saying? (Usually objectionable.) So you’re unhappy. Why tell me. You must have mistaken me for someone who gives a shit
See also: give, have, mistaken, must, shit, someone, who
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions
See also:
  • don't beat a dead horse
  • don't go there
  • don't get me wrong
  • cast (one's) lot in with (someone or something)
  • cast lot in
  • (I) don't care if I do
  • don't care if I do
  • all in (one's) head
  • come it (with one)