how can/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. © 2022 Farlex, Inc, all rights reserved.

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. © 2002 by The McGraw-Hill Companies, Inc.

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 © Farlex 2017
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