what does the saying "on your six's" come from
The saying comes from looking at an analog clock face. Assuming you are at the center looking at 12 o'clock, "6" is at your back. "On your six" means "having your back".
The saying "on your six" is a phrase used in reference to the position of the person who says it. They may mean they are right behind you or they are watching your every move. It is commonly used in the military.