What does "on your six" mean?

6 or six - Used as a slang term by military and civilian pilots and aviators as a shortened form of "6 o'clock" or directly behind you. "Six" may also be substituted for the hindquarters, buttocks, butt, or any other slang term commonly used by non-military aviator. "Six" may also be used in the phrase "Check your six," which can be used as a warning or as a greeting or salutation among pilots: Check your back.

e.g., "He's on my 6. " "You had better watch out, or I'll be on your six faster than a surface-to-air missle."

http://english.stackexchange.com/questions/157517/on-your-six-and-watch-your-six

"on your six" = "behind you"

It can be used as an Insult. Behind, etc. Except it is a bit differently said.....As in "In".