Etymology
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loco (adj.)
"mad, crazy," 1844, American English, from Spanish loco (adj.) "insane," of uncertain origin, perhaps from Arabic lauqa, fem. of 'alwaq "fool, crazy person." Loco-weed was the name given to species of western U.S. plants that cause cattle and horse diseases that make them stagger and act strangely. But the adjective seems to be the older word.
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in (adj.)
"that is within, internal," 1590s, from in (adv.). Sense of "holding power" (the in party) first recorded c. 1600; that of "exclusive" (the in-crowd, an in-joke) is from 1907 (in-group); that of "stylish, fashionable" (the in thing) is from 1960.
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in (adv., prep.)

a Middle English merger of Old English in (prep.) "in, into, upon, on, at, among; about, during;" and Old English inne (adv.) "within, inside," from Proto-Germanic *in (source also of Old Frisian, Dutch, German, Gothic in, Old Norse i), from PIE root *en "in." The simpler form took on both senses in Middle English.

Sense distinction between in and on is from later Middle English, and nuances in use of in and at still distinguish British and American English (in school/at school). Sometimes in Middle English shortened to i.

The noun sense of "influence, access (to power or authorities)," as in have an in with, is first recorded 1929 in American English. to be in for it "certain to meet with something unpleasant" is from 1690s. To be in with "on friendly terms with" is from 1670s. Ins and outs "intricacies, complications of an action or course" is from 1660s. In-and-out (n.) "copulation" is attested from 1610s.

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loco-foco (n.)
also locofoco, American English, said to date from 1834 in the sense "self-igniting cigar or friction match," of obscure origin. The first element is apparently a misapprehension of the loco- in locomotive ("a word just then becoming familiar" [Century Dictionary]) as "self-, self-moving-." The second element is perhaps a jingling reduplication of this, or somehow from Spanish fuego "fire."

Better remembered, if at all, as a political term: During a heated Democratic party meeting in Tammany Hall c. 1835, the opposition doused the gaslights to break it up, and the radical delegates used loco-foco matches to relight them. When it was publicized, the name loco-foco entered U.S. political jargon (by 1837) and down to the Civil War was applied, usually disparagingly, to a radical faction of the Democratic Party (but by the Whigs to all Democrats).
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loco-weed (n.)
plant of the U.S. West, noted for its effect on cattle and horses that ate it, 1877; see loco (adj.) "crazy" + weed (n.).
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in-flight (adj.)
also inflight, "during or within a flight," 1945, from in (prep.) + flight.
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in-store (adj.)
also instore, 1954, from in (prep.) + store (n.). In Middle English, instore was a verb meaning "to restore, renew," from Latin instaurare.
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shoo-in (n.)
"easy winner" (especially in politics), 1939, from earlier sense "horse that wins a race by pre-arrangement" (1928); the verb phrase shoo in in this sense is from 1908; from shoo (v.) + in (adv.).
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trade-in (n.)
1917, in reference to used cars, from verbal phrase, from trade (v.) + in (adv.).
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in-patient (n.)
also inpatient, "person lodged and fed, as well as treated, at a hospital or infirmary," 1760, from in (adj.) + patient (n.). As an adjective by 1890.
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