Later for American troops it was sometimes used, with a different intonation, to describe the vacant expression on the face of dope-heads just serving out their time
This stare can be useful for finding attractive people as well. You will find this useful when you need a wingman (or wingwoman) for a night, or twenty.
Dude #2 "Yeah, about 1000 yards before you!"
Dude #1 "Dude! You got the 1000 yard stare!!" (Bows)
Girl #1 "Yeah, my boyfriend found me 1000 yards away.. How did you and (points to non-listening boyfriend) meet?
Girl #2 (quietly) "I caught him staring at my boobs from 2 feet away.."