Watch your ass

"watch your ass'"
"watch your tail"
"watch your six"

Dates from WWII, 1917, likely first British then universally from WWII aviators and military. The everyone picked it up.

Thanks, Joe. I should mention that it's helpful if, when posting a comment, you allow all previous comments to remain, posting your own comment at the end.
Incidentally, I'm not familiar with "watch your six." Perhaps you can remedy my ignorance.
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