“It originated in World War One. Fighter pilots referenced the rear of the airplane as the six o’clock position. If you think about a clock face, and you’re standing in the middle, twelve o’clock is in front of you, three is to your right, nine to your left, and six is behind you. On a battlefield, your ‘six’ is the most vulnerable position because you don’t have eyes in the back of your head. So when someone says they have your six, they mean they’re watching your back.”
