Norimatsu Onishi, "Bomb by Bomb, Japan Sheds Military Restraints", NYT 7/23/2007:
To take part in its annual exercises with the United States Air Force here last month, Japan practiced dropping 500-pound live bombs on Farallon de Medinilla, a tiny island in the western Pacific's turquoise waters more than 150 miles north of here.
The pilots described dropping a live bomb for the first time — shouting "shack!" to signal a direct hit — and seeing the fireball from aloft.
This use of shack is not in the OED, but Grant Barrett has it covered:
shack n. a direct hit on a target by a bomb or missile.
But Grant's earliest citation is from 1998, and I know from personal experience that it goes back to Vietnam in the 1960s. Here's an interview with Roger Preu from the Stamford Historical Society, about his experiences in WW II, showing that it goes back to 1943, at least as a term for the target in bombing practice:
And if you hit the target, the shack it was called, it was built up where you'd see it.
And though shack probably was a noun to start with, you won't be surprised to learn that it's used as some other part of speech as well. The example from today's NYT suggests that it's often a kind of specialized interjection. Here's an article from a U.S. Air Force Source ("Raptor drops first bomb", 10/21/2005) where it's used as a verb:
After watching the first bombing flight through a live television feed, the colonel said he could tell it was a successful event, but not where the bombs hit.
Hill's Weapons Systems Evaluation Program operators verified the bombs not only hit the targets, they "shacked" them.
"That's a fighter-pilot term for when you hit a target dead center -- a bull's-eye." said Capt. Shawn Anger, 43rd FS air-to-ground weapons chief. "Hit criteria will vary depending on the size of the target and the munitions, but when you put the bomb directly in the center of the target it's a shack."
And here (Mark Bowden, "The Kabul-ki Dance", The Atlantic 11/2002) it's an adverb:
The bomb hit "shack on," or dead center, and the SAM launcher vanished in a satisfactory black splash on the monitor.
[Update -- Benjamin Zimmer located this from the Nov. 1956 issue of Boeing Magazine (misdated as 1934 by Google Books, following its usual unfortunate practice of dating all issues of a serial in terms of the earliest issue):
"It's a shack!" someone yelped. "Shack" is a bombing man's term for bull's-eye, dating from the days when the usual target on a bombing range was indeed a shanty built of boards. Sure enough, it was a perfect hit on a hat-sized target at Springfield, Massachusetts — the only "shack" of the day among the heavy bombers: the all-jet B-52s and the piston-powered B-36s.
...
And at Montreal, third target city, it was a storage tank. But not the whole tank. Merely the geometrical center of a circle formed by a small railing atop the tank. Hit that, dead center, and you've got a "shack."
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[Update -- Jim Gordon writes:
The usage was first and foremost by bomber crews of the 1940s, because fighters and dive-bombers used flat targets. Ben Zimmer's citation has the original usage and the using population. The WWII Norden bombsight worked best when the target was a structure with some vertical dimension, rather than being a flat surface on the ground. Thus, the bombing ranges used to train B-17, B-24 and B-29 crews had rudimentary shacks built as/at the aiming point for dummy or practice bombs. (I'm not sure whether smaller Air Corps bombers used them, nor do I know which Navy bombers carried them -- the Norden bombsight was invented for the Navy.)
As an Air Force brat, I regularly heard and read the term used by SAC aircrew in the early 1950s, and by the Air Defense Command pilots and navigators who flew "aggressor" missions, simulating the Russian bombers that we all expected at any moment, to test the air defense system of the mid-1950s. The measurement equipment of the 1950s made it unnecessary to drop even a dummy "shape" (bomb), instead using a rudimentary computer in a truck-transportable box or shack. If one looked at SAC publications of the 1950s, intended to propagandize the rest of the Air Force and other services, reports of the annual bombing competitions would include use of "shack."
When I served in the Air Force 1966-1970, the term had shifted away from bomber-crew use as atomic bombs became the weapon of the day and iron bombs were disdained in SAC. With atomic bombs, the targets were much larger "target islands" and precision was relative. The term was largely replaced by references to the "circular error" (a term much used by the end-of-WWII Strategic Bombing Survey of European targets), or by references to the distance of impact from the desired ground zero (DGZ); If they hit the DGZ, they had "zero error" or "zero C.E.P." (sic -- The abbreviation for the pre-drop estimate of the expected "Circular Error, Probable" became the term for the result.) As fighters were equipped with more complex, computing bombsights, late-Vietnam era, the shack term moved into use in the fast-movers' culture.
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Posted by Mark Liberman at July 23, 2007 11:59 AM