Frozen,
Where it started, I can not tell u, but the Military and LE both use the "Clock" terminology extensively. It relates to position. IE: 12 oclock is straight ahead, 3 oclock is 90 degrees to your right, 6 oclock is another 90 degrees going to your immediate rear, or "Check your Six". All the numbers on a watch can be used to denote a position or direction.

Krell