Watch your six 

Watch out for danger behind your back (like you might get jumped or something).

Watch your back.

Keep an extra set of eyes on the back of your head.

Dude, be careful man. Watch your six.

Watch your six by Brent Ninja October 11, 2012

Watch your six 

Watch your back.
Keep an eye on what could be behind you.
Turn around and look.

Yo bro, watch your six!

Watch your six by K!ngFox July 10, 2021

cover/watch your six 

Military term for watching/covering your back/rear side

Stay frosty and cover/watch your six!

cover/watch your six by gunshy77 May 11, 2014