I watched Grey's Anatomy, an American TV drama. Dr. Grey, a young female Intern made a mistake on her patient. She might have been kicked out, but Dr. Burka spoke for her. Whe she was out, she told her friends, "Burka saved my ass".
I found a native speaker used "a$$" when he refered someone was an ass.
Is it rude to say someone saves my ass, especially for a woman?