look up any word, like donkey punch:
 
1.
directly behind you. from the military designation of location where 12:00 is directly in front of you, 3:00 is to your right, 9:00 is to your left, etc.
Hang on, we've got two Japanese Zeroes on our six! I'll try to shake them!
by fizzle April 07, 2004
 
2.
Watching your every move, after you, hunting you down.
If you dare mess up this deal I'will be on your six.
by Joseph Wambua May 23, 2006