A colorful term meaning approximately "hick", "idiot", or "stupid person".
"You think I'm a complete backbirth?"
"He’s a total backbirth."
(n.) A pejorative whose recipient the utterer believes to have been born through his mother's anus; an assbaby. May be applied equally to those who exhibit stupid, sociopathic, or otherwise fecal behavior.
"Do you think I'm a total backbirth?"
"Dude, quit being such a damn backbirth!"