backbite

To say malicious, nasty, or catty things about someone. He's a mouthy kid, known to backbite about anyone who tries to tell him what to do. Honey, I'm sure your friends wouldn't backbite about you to the rest of the school. Stu always backbites, but I don't pay attention to him anymore. I refuse to be miserable like him.

backbiting

The act of saying malicious, nasty, or catty things about someone. Don't let her backbiting deter you—she complains about everyone and everything all the time. If you keep backbiting like that, you're going to end up with detention, mister! Oh, I don't pay any attention to Stu's backbiting anymore. I refuse to be miserable like him.