MX stands for Missile Experimental, which hardly inspires confidence. So President Reagan let it be known he wanted to rename it. The National Security Council suggested Themis, for the Greek goddess of order and justice. Defense Secretary Caspar Weinberger's choice was Guardian Missile. National Security Adviser William Clark proposed Peacemaker, after the Colt .45 that belonged to his grandfather and now hangs on a wall in his office. This caught the President's fancy, but White House aides pointed out that the name Peacemaker might violate a copyright or, even worse, be twisted into a pejorative: Pacemaker. Reagan finally compromised on Peacekeeper. Not that it mattered: at week's end everyone was still calling it the MX.