Someone used the expression "watch your 6" in a thread recently, and when I did a Google search to find the definition, I came across this blog on personal safety:

Qui Custodes

The content is primarily about how to act in typical urban situations, and is only updated every so often. I found some useful stuff.

The author is a lawyer specializing in martial arts liability. What an interesting career path that must have been!

"Watch your six" means look behind you. Its a fighter pilot' expression refering to ACM. The classic perfect kill is from behind, in the bandits blind spot. Used to murderious effect by Richtofen, Imaman, Ball and various others. Get as high as possible, come down as fast as possible, Get as close as possible, kill. Then be gone as fast as possible.

Did that perhaps come out of the thread on the Las Vegas punks putting a beatdown on the guy in a cart? I think that may have been me. Nice site you found, and interesting. I wish he had some general answers there, but as a lawyer I'm sure he has to avoid providing what could be construed as legal advice.

"check 6" is more accurate.

Keep your head on a swivel -- looking for "bad guys".

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

The term "watch your six" or "check your six" means "watchout for your ass". This is also used in a military sense where the political may be nearly as dangerous as the tactical.

"There is nothing so frightening as ignorance in action."

That, as a general rule, means any where the fourth estate might be watching and passing judgement.