on your six

on (one's) six

Directly behind someone. Sometimes used as an imperative to look behind one. A reference to the numbers on the face of a clock, with 12 being the direction one is looking. Someone provide cover fire, there are snipers on our six! On your six, Tom—Mary's here, and she looks angry.
See also: on, six

on your six

phr. [look] behind you. ) (see also Behind you!.) Look out! On your six!
See also: on, six