Etymology
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April fool (n.)
1680s; April-gowk (from Old Norse gaukr "a cuckoo") is a northern variant. April Fool's Day customs of sending people on false errands seem to have come to England from France late 17c.; originally All Fool's Day (1712). In Cumberland, Westmorland and northern parts of Lancashire and Yorkshire, however, May 1 was the day for hoaxing, and the fool was a May gosling. That custom was first attested 1791.
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Agent Orange (n.)
powerful defoliant used by U.S. military in the Vietnam War, reported to have been used from 1961; so called from the color strip on the side of the container, which distinguished it from Agent Blue, Agent White, etc., other herbicides used by the U.S. military; see agent (n.). Banned from April 1970.
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Arbor Day 
day set aside in U.S. "for planting forest trees to make lumber for the generations yet to come" ["Congressional Record," June 1892], first celebrated April 10, 1872, in Nebraska (a largely treeless state), the brainchild of U.S. agriculturalist and journalist J. Sterling Morton (1832-1902). From Latin arbor, arboris "tree" (see arbor (n.2)).
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hands down (adv.)

to win something hands down (1855) is from horse racing, from a jockey's gesture of letting the reins go loose in an easy victory.

The Two Thousand Guinea Stakes was not the best contested one that it has been our fortune to assist at. ... [T]hey were won by Meteor, with Scott for his rider; who went by the post with his hands down, the easiest of all easy half-lengths. Wiseacre certainly did the best in his power to spoil his position, and Misdeal was at one time a little vexatious. [The Sportsman, report from April 26, 1840]

Ancient Greek had akoniti "without a struggle, easily," from akonitos (adj.), literally "without dust," specifically "without the dust of the arena."

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hoochy koochy (n.)

also hoochie-coochie, hootchy kootchy, "erotic suggestive women's dance" (involving a lot of hip-grinding), 1898, of obscure origin, usually associated, without evidence, with the Chicago world's fair of 1893 and belly-dancer Little Egypt (who might not even have been there), but the word itself is attested from 1890, as the stage name of minstrel singer "Hoochy-Coochy Rice," and the chorus of the popular minstrel song "The Ham-Fat Man" (by 1856; see ham (n.2)) contains the nonsense phrase "Hoochee, kouchee, kouchee."

To-day, however, in place of the danse du ventre or the coochie-coochie we have the loop-the-loop or the razzle-dazzle, which latter, while not exactly edifying at least do not serve to deprave public taste. ["The Redemption of 'Old Coney,'" in Broadway Magazine, April 1904]
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color-blindness (n.)

also colour-blindness, "incapacity for perceiving certain colors due to an absence or weakness of the sensation upon which the power of distinguishing them depends," 1844, the native word, used in England instead of French daltonisme (by 1828), after English chemist John Dalton (1766-1844), who published a description of it in 1794. From color (n.) + blindness.

The continental philosophers have named it Daltonism, a name which has been strongly objected to by almost every English writer who has discussed the subject, on the ground of the inexpediency and undesirableness of immortalizing the imperfections or personal peculiarities of celebrated men by title of this kind. ... The name "Color-Blindness," proposed by Sir D. Brewster, seems in every respect unexceptionable. [Littell's Living Age, vol. v, April 1845]

Noted as inexact (very few people who can see are blind to all color), "the term is applied with much laxity to any constitutional inability to discriminate between colours" [OED]. In figurative use, with reference to race or ethnicity, it is attested from 1866, American English. Related: color-blind (adj.), which is attested from 1854.

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