Etymology
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system (n.)
1610s, "the whole creation, the universe," from Late Latin systema "an arrangement, system," from Greek systema "organized whole, a whole compounded of parts," from stem of synistanai "to place together, organize, form in order," from syn- "together" (see syn-) + root of histanai "cause to stand," from PIE root *sta- "to stand, make or be firm."

Meaning "set of correlated principles, facts, ideas, etc." first recorded 1630s. Meaning "animal body as an organized whole, sum of the vital processes in an organism" is recorded from 1680s; hence figurative phrase to get (something) out of one's system (1900). Computer sense of "group of related programs" is recorded from 1963. All systems go (1962) is from U.S. space program. The system "prevailing social order" is from 1806.
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weapon (n.)

Old English wæpen "instrument of fighting and defense, sword," also "penis," from Proto-Germanic *wēipna- (source also of Old Saxon wapan, Old Norse vapn, Danish vaaben, Old Frisian wepin, Middle Dutch wapen, Old High German wafan, German Waffe "weapon"), a word of unknown origin with no cognates outside Germanic; possibly a substratum word.

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Dewey Decimal system (n.)

library classification system that organizes information into 10 broad areas subdivided numerically into progressively smaller topics, by 1885, named for Melvil Dewey (1851-1931) who proposed it 1876 while acting librarian of Amherst College. He also crusaded for simplified spelling and the metric system.

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Luftwaffe 
air arm of the German Wehrmacht in the World War II era, 1935, from German Luftwaffe, literally "air-weapon," from Luft (see loft (n.)) + Waffe (see weapon (n.)).
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DOS 

"computer operating system using a disk storage device," 1967, acronym of disk operating system.

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automatic (n.)

1902, "automatic weapon," from automatic (adj.). Meaning "motorized vehicle with automatic transmission" is from 1949.

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quarterstaff (n.)

also quarter-staff, 1540s (quarter-stroke "stroke with a quarterstaff" is attested from early 15c.), an old weapon formed from a stout pole, six to eight feet long (six-and-a-half sometimes is given as the standard length), tipped with iron, formerly a weapon characteristic of the English peasantry. From staff (n.); the quarter in it is of uncertain signification. According to one theory, favored by fencing manuals, etc., it likely is in reference to operation of the weapon:

It was grasped by one hand in the middle, and by the other between the middle and the end. In the attack the latter hand shifted from one quarter of the staff to the other, giving the weapon a rapid circular motion, which brought the ends on the adversary at unexpected points. [Century Dictionary]

Linguists tend to prefer an explanation from woodcutting, perhaps a reference to a cut of lumber known as a quarter, but contemporary evidence is wanting for either conjecture.

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systematize (v.)
"make into a system," 1764, from French systématiser or a native formation from system (Greek stem systemat-) + -ize. Related: Systematized; systematizing.
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slasher (n.)
1550s, "a bully, a fighter;" 1815, "weapon for slashing," agent noun from slash (v.). As "violent movie" by 1978.
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