***** NOT A TEACHER *****
(1) I think it means something like:
"How dare you say that!" or "How dare you do that!"
"I cannot believe that you would actually say that!" or "I cannot believe that you would
actually do that!"
(2) Here is a true story:
Some years ago, an Australian gentleman wrote to the editor of an important
magazine in London, England. His letter was something like:
Dear Sir: Please stop the use of British spelling. Please spell all
words in the way Americans do."
(3) The editor, of course, was shocked by this rude "order." The editor replied with
one short sentence: What a nerve!