How could you?

how could you

An expression of shock, dismay, and displeasure over another's actions. How could you forget our anniversary? We've only been married a year! So you told the boss about my personal struggles to get ahead. Wow, how could you?
See also: could, how
Farlex Dictionary of Idioms.

How could you (do something)?

How could you bring yourself to do a thing like that? (No answer is expected.) Looking first at the broken lamp and then at the cat, Mary shouted, "How could you do that?" Tom: Then I punched him in the nose. Rachel: Oh, how could you?
See also: could, how
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

how ˈcan/ˈcould you!

(spoken) used to show that you strongly disapprove of somebody’s behaviour or are very surprised by it: Ben! How could you? After all they’ve done for us! Ugh! How can you eat that stuff?
See also: can, could, how
Farlex Partner Idioms Dictionary
See also:
  • how can/could you!
  • how could you
  • under the thumb of (someone)
  • under thumb
  • under (one's) thumb
  • under somebody's thumb
  • under someone’s thumb
  • under someone's thumb
  • in bad odor
  • in bad odour