'Vanderpump Rules' Star Peter Madrigal -- Stassi Had to Go ... She Called Lisa the O-Word! (VIDEO)

"Vanderpump Rules" had to jettison Stassi Schroeder from the show because she made one FATAL mistake -- calling Lisa Vanderpump ... OLD.

That rationale comes courtesy of co-star Peter Madrigal, who took the opportunity Wednesday to absolutely trash his now former co-star.

Check it out ... Madrigal is downright angry about Stassi going after the boss lady, and straight up excited she's gone. His attempt to give her a friendly-ish kiss off is cute and transparent as hell.

As TMZ first reported, Stassi's attitude is what did her in ... and she's not only off the show, she's out at all of Lisa's restaurants too.