Etymology
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blue moon (n.)

"a long time," 1821, often in phrases indicating something rarely occurring. Compare at the Greek calends (from an ancient Roman phrase alluding to the fact that the Greeks had nothing corresponding to the Roman calends), and the native in the reign of Queen Dick and Saint Geoffrey's Day "Never, there being no saint of that name," reported in Grose (1788). Nevermass "date which never comes" is from 1540s. Blue moon is suggested earliest in this couplet from 1528:

Yf they say the mone is blewe,
We must beleve that it is true.

Though this might refer to calendrical calculations by the Church. Thus the general "rareness" sense of the term is difficult to disentangle from the specific calendrical one (commonly misinterpreted as "second full moon in a calendar month," but actually a quarterly calculation). In either case, the sense of blue here is obscure. Literal blue moons do sometimes occur under extreme atmospheric conditions.

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blue laws 

severe Puritanical code said to have been enacted mid-17c. at the founding of New Haven and Connecticut colonies, 1781; of uncertain signification, perhaps from the notion of coldness, or from one of the figurative senses in blue (adj.1). Blue was the color adopted by 17c. Scottish Covenanters (in contradistinction to the royal red) and hence the color for a time acquired an association with strictness in morals or religion. Or perhaps connected to bluestocking in the sense of "puritanically plain or mean" (see bluestocking, which is a different application of the same term); the parliament of 1653 was derisively called the bluestocking parliament.

The assertion by some writers of the existence of the blue laws has no other basis than the adoption by the first authorities of the New Haven colony of the Scriptures as their code of law and government, and their strict application of Mosaic principles. [Century Dictionary]

Long, detailed lists of them often are given, but the original reference (in an anonymous history of Connecticut printed in London during the Revolution) says they were so-called by the neighboring colonies, "were never suffered to be printed," and then gives its own long list of them in quotations. The common explanation (dating to 1788) that they were written on blue paper is not now considered valid.

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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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sacre bleu (interj.)

an English notion of a stereotypical French oath, 1869, from French sacré bleu, literally "holy blue," a euphemism for sacré Dieu (1768), "holy God." From Old French sacrer, from Latin sacrare "to make or declare sacred" (see sacred). Such things are never idiomatic. In an old French-language comic set in the U.S. Wild West, the angry cowboys say "Bloody Hell!"

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silver lining (n.)

a "bright side" which proverbially accompanies even the darkest trouble; by 1843, apparently from oft-quoted lines from Milton's "Comus," where the silver lining is the light of the moon shining from behind the cloud.

Was I deceived? or did a sable cloud
Turn forth her silver lining on the night?
I did not err, there does a sable cloud,
Turn out her silver lining on the night
And casts a gleam over this tufted grove.

To which Thomas Warton added the commentary: "When all succour ſeems to be lost, Heaven unexpectedly presents the ſilver lining oſ a ſable cloud to the virtuous."

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black swan (n.)

proverbial for "something extremely rare or non-existent" (late 14c.) is from Juvenal ["Sat." vi. 164], but the real thing turned up in Australia (Chenopsis atratus).

"Do you say no worthy wife is to be found among all these crowds?" Well, let her be handsome, charming, rich and fertile; let her have ancient ancestors ranged about her halls; let her be more chaste than all the dishevelled Sabine maidens who stopped the war—a prodigy as rare upon the earth as a black swan! yet who could endure a wife that possessed all perfections? I would rather have a Venusian wench for my wife than you, O Cornelia, mother of the Gracchi, if, with all your virtues, you bring me a haughty brow, and reckon up Triumphs as part of your marriage portion. [Juvenal]

Blue dahlia also was used 19c. for "something rare and unheard of."

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Memorial Day 

"day on which a memorial is made," by 1819, of any anniversary date, especially a religious anniversary; see memorial (adj.). As a specific end-of-May holiday commemorating U.S. war dead, it began informally in the late 1860s and originally commemorated the Northern soldiers killed in the Civil War. It was officially so called by 1869 among veterans' organizations, but Decoration Day also was used. The Grand Army of the Republic, the main veterans' organization in the North, officially designated it Memorial Day by resolution in 1882:

That the Commander-in-Chief be requested to issue a General Order calling the attention of the officers and members of the Grand Army of the Republic, and of the people at large, to the fact that the proper designation of May 30th is Memorial Day and to request that it may be always so called. [Grand Army Blue Book, Philadelphia, 1884]

The South, however, had its own Confederate Memorial Day, and there was some grumbling about the apparent appropriation of the name.

The word "Memorial" was adopted by the Maryland Confederates shortly after the war, and has been generally used throughout the South. It is distinctively Confederate in its origin and use, and I would suggest to all Confederate societies to adhere to it. The Federals' annual day of observance is known as "Decoration Day," having been made so by an act of Congress, and the 30th day of May named as the date. In Maryland there is annually a Decoration Day and a Memorial Day. The two words are expressive not only of the nature of the observance, but also of the people who participate therein. [Confederate Veteran, November 1893]
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