Watch your six!

[pinstripe]

Why in American movies do they say "Watch your six" when they are referring to watching someone's back?

[NazNomad]

6 o' clock ...

... before the days of advanced in-cockpit radar and heads-up displays were described in relation to an imaginary horizontal clockface, with 12 o'clock being directly ahead of the pilot, and 6 o'clock directly behind him.

[Clanad]

Even today, when Air Traffic Control (ATC) advises one aircraft of other traffic in the vicinity the report will go something like this:

ATC: "Buzzabout 1211, traffic at 11:00 O'clock, 6 miles, flight level 350 (pronounced zero) west bound"...
Buzzabout 1211: "Rodger, were looking, negative contact"...
(Or alternatively)
Buzzabout 1211: "Rodger, traffic in sight"...

[heathfield]

ATC: Flybynite 273, do you see JAL 128 at 11:00 O'clock, 10 miles?
Flybynite 273: Er, Negative.

ATC: Flybynite 273, do you see JAL 128 at 12:00 O'clock, 8 miles?
Flybynite 273: Still looking. Negative contact.

ATC: Flybynite 273, do you see JAL 128 at 1:00 O'clock, 5 miles?
Flybynite 273: Negative, control. I can't see that goddam Nip anywhere!
JAL 128: Look into the sun, Yankee dog!!