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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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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loco- 
word-forming element meaning "from place to place," from combining form of Latin locus "a place" (see locus).
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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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in loco parentis 
legal Latin, 1640s in English, literally "in the place of a parent," from loco, ablative of locus "a place" (see locus (n.)) + parentis, genitive of parens "parent" (see parent (n.)).
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locomotion (n.)
1640s, "action or power of motion," from Latin loco "from a place" (ablative of locus "a place;" see locus) + motionem (nominative motio) "motion, a moving" (see motion (n.)). From 1788 as "movement from place to place."
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locomotive (adj.)
1610s, "pertaining to movement," from French locomotif, from Latin loco "from a place" (ablative of locus "place;" see locus) + Late Latin motivus "moving" (see motive).

From 1650s as "moving from place to place;" by 1814 as "having the power of moving by itself. The noun meaning "engine which travels on rails by its own power" is from 1829, short for locomotive engine, which is attested from 1814. A locomotive engine used without rails was a traction engine, which became tractor.
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loc. cit. 
abbreviation of Latin loco citato or locus citatus "in the place (already) cited;" hence, "in the book that has been previously quoted." See locus, cite. In use in English books by 1704.
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instead (adv.)
1590s, contraction of Middle English prepositional phrase ine stede (early 13c.; see stead), itself a loan-translation of Latin in loco (French en lieu de). Typically written as two words until mid-17c.
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locomotor (adj.)
1870, "of or pertaining to locomotion;" probably based on locomotion (as motor/motion). Earlier as a noun, "something with locomotive power" (1822). Related: Locomotory; locomotorial.
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