Donald Sterling threatened to MURDER Shelly Sterling's attorney, Pierce O'Donnell, during a phone call earlier this month ... this according to Shelly who will try to smack Donald with a restraining order later today ... TMZ Sports has learned. Sources connected to the situation tell us ... when Shelly's lawyers go to court this morning, they will tell the judge that Donald called Pierce on June 9th and threatened to "sue everyone" connected to the sale of the L.A. Clippers. We're told ... Shelly says during the call, Donald told Pierce that he will "take you out" -- which Shelly and Pierce both believe is a direct threat on his life.