Etymology
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air force (n.)
1917, from air (n.1) + force (n.); first attested with creation of the Royal Air Force. There was no United States Air Force until after World War II. The Air Corps was an arm of the U.S. Army. In 1942, the War Department reorganized it and renamed it Army Air Forces. The National Security Act of 1947 created the Department of the Air Force, headed by a Secretary of the Air Force, and the U.S.A.F.
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force majeure (n.)
1883, French, "superior strength."
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cop out 

by 1942, noun ("a cowardly escape, an evasion") and verb ("sneak off, escape, give up without trying"), American English slang, perhaps from cop a plea (c. 1925) "plead guilty to lesser charges," which is probably from northern British slang cop "to catch" (a scolding, etc.); as in cop a feel "grope someone" (1930s); see cop (v.). Sense of "evade an issue or problem" is from 1960s.

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hang out (v.)
c. 1400, intransitive (as of the tongue, from the mouth); transitive use by 1560s; see hang (v.) + out (adv.). Colloquial meaning "to be found" is recorded from 1811, "in allusion to the custom of hanging out a sign or 'shingle' to indicate one's shop and business" [Century Dictionary]. As a noun (often hangout) "residence, lodging" attested from 1893; earlier "a feast" (1852, American English).
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go out (v.)
early 13c., "leave home," from go (v.) + out (adv.). Meaning "become extinct, expire" is from c. 1400.
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make out (v.)

c. 1600, "get along, succeed," from make (v.) + out (adv.). Sense of "obtain a clear understanding of" is from 1640s; that of "discern or discover visually" is by 1754; sense of "have sexual relations with" is attested by 1939.

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pass out (v.)

"lose consciousness," 1915, from pass (v.) + out. Probably a weakened sense from earlier meaning "to die" (1899). Meaning "to distribute" is attested from 1926. Related: Passed out.

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tour de force (n.)
"feat of strength," 1802, French; see tour (n.) + force (n.).
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bug off (v.)
"leave quickly," by 1956, perhaps from bugger off (see bugger (v.)), which chiefly is British (by 1920s) but was picked up in U.S. Air Force slang in the Korean War. Also see bug (v.3). To bug out "leave quickly, scram" is from 1953.
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back up (v.)
1767, "stand behind and support," from back (v.) + up (adv.). Meaning "move or force backward" is by 1834. Of water prevented from flowing, by 1837.
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