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!
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Watching your every move, after you, hunting you down.
If you dare mess up this deal I'will be on your six.
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I'm following you or I'm behind you; more broadly, I'll back you up/take your side. Also, "I'm on your six". Refers to the six o'clock position relative to the listener (behind).
Car Phone 1: Where are you? I don't see your car.
Car Phone 2: Look in your rearview mirror; I'm on your six.
Joe: Dude, I need some heavy backup for tomorrow's meeting!
Jack: Don't worry man; I got your six.
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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.
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A term often used and popularized by the military to check at your 6 o'clock position. Which would be right behind you at any given point.
Sergeant: Private, check your six and tell me if you see any bogeys.
Private: Yes sir!
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Watch your back.
Keep an eye on what could be behind you.
Turn around and look.
An expression used to inform someone that there's a target at their 6 o' clock position.
1: "Check your six."
2: "... Damn."
1: "Told ya."
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