Should be "stobor"
Robert Heinlein used it thusly:
(DANG IT!! WHO TOOK MY COPY OF TUNNEL IN THE SKY!!)
...hold on, have to ebook this one...
SOLO SURVIVAL TEST
Recall Instructions
1. You must pass through the door ahead in the three minutes allowed you before another candidate is started through. An overlapping delay will disqualify you.
2.Recall will be by standard visual and sound signals. You are warned that the area remains hazardous even after recall is sounded.
3.The exit gate will not be the entrance gate. Exit may be as much as twenty kilometers in the direction of sunrise.
4. There is no truce zone outside the gate. Test starts at once. Watch out for stobor. Good luck!
...basically, a warning to watch out for the unknown. Stobor is robots spelled backwards, obviously.
Also interesting about Tunnel In The Sky: Heinlein invented the Camelback in it:
Rod put on his vest pack of rations and sundries, fastened his canteen under it. It was a belt canteen of flexible synthetic divided into halflitre pockets. The weight was taken by shoulder straps and a tube ran up the left suspender, ending in a nipple near his mouth, so that he might drink with out taking it off. He planned, if possible, to stretch his meager supply through the whole test, avoiding the hazards of contaminated water and the greater hazards of the water hole assuming that fresh water could be found at all.
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Member of the toughest, meanest, deadliest, most unrelenting -- and ablest -- form of life in this section of space, a critter that can be killed but can't be tamed. --Robert A. Heinlein