George Brett -- THREATENS AUTOGRAPH SEEKER ... 'I'll Shoot Your F**king Ass'

MLB great George Brett went BALLISTIC on an autograph seeker Wednesday ... threatening to "f**king shoot" the guy if he ever saw him again, and it was all captured on video.

Brett -- who still works for the Royals organization -- had just arrived to Kansas City International Airport yesterday when he was confronted by Brandon Farrens ... who asked for an autograph.

Brett obliged ... saying it was the last time he'd ever sign for the guy -- and then he added, "If I ever see you f**king again, I'll shoot your f**king ass."

Farrens responded, "What an a**hole."

Brett goes back toward the camera -- and that's when the shot ends.