Etymology
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double date (n.)

"date involving two couples," by 1922, American English, from double (adj.) + date (n.3). As a verb by 1938. Related: Double-dating.

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blind date (n.)
by 1921, U.S. college student slang, from blind (adj.) + date (n.3). Earliest attested use is in reference to the person; of the event by 1925.
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light bulb (n.)
also lightbulb, 1884, from light (n.) + bulb (n.). Changing one as figurative of something easy to do is from 1920s; jokes about how many of a certain type it takes to change one date from 1971.
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old hat (adj.)

"out of date," 1911, from old + hat. As a noun phrase, however, it had different sense previously. The "Classical Dictionary of the Vulgar Tongue" (1796) defines it as, "a woman's privities, because frequently felt."

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Underground Railroad (n.)
"network of U.S. anti-slavery activists helping runaways elude capture," attested from 1847, but said to date from 1831 and to have been coined in jest by bewildered trackers after their slaves vanished without a trace. Originally mostly the term for escape networks in the (then) western states of the U.S.
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tea party (n.)
1772, from tea + party (n.). Political references to tea party all trace to the Boston tea party of 1773 (the name seems to date from 1824), in which radicals in Massachusetts colony boarded British ships carrying tea and threw the product into Boston Harbor in protest against royal taxation. It has been a model for libertarian political actions in the U.S. (generally symbolic), including citizen gatherings begun in early 2009 to protest government spending.
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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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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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main line (n.)

"principal line of a railway," 1841, from main (adj.) + line (n.). Meaning "affluent area of residence" is by 1917, originally (with capitals) that west of Philadelphia, from the "main line" of the Pennsylvania Railroad which added local stops to a string of backwater towns west of the city late 19c. that helped turn them into fashionable suburbs.

The Main Line, Philadelphia's most famous suburban district, was deliberately conceived in the 1870's and 1880's by the [Pennsylvania] Railroad, which built high-toned housing developments, ran hotels, more or less forced its executives to plunk their estates out there, and created a whole series of somewhat spurious Welsh towns along the railroad tracks. ... Now everybody assumes these all date from 1682, like the Robertses; but as Chestnut Hill people like to say, "nobody but Welsh peasants lived on the Main Line till the Railroad built it up." [Nathaniel Burt, "The Perennial Philadelphians," 1963]

The original station stops were, in order out from the city, Overbrook, Merion, Narberth, Wynnewood, Ardmore, Haverford, Bryn Mawr, Paoli. The train line for commuters along it is the Paoli Local.

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dumdum (n.)

type of metal-cased bullet which expands on impact, 1897, named for Dum-Dum arsenal in Bengal. British army soldiers made them to use against fanatical charges by tribesmen. Outlawed by international declaration, 1899. The place name is literally "hill, mound, battery," cognate with Persian damdama.

It was to stop these fanatics [Ghazi] — and that firstclass fighting man Fuzzy-Wuzzy, of the Soudan and of Somaliland—that the thing known as the Dum-Dum bullet was invented. No ordinary bullet, unless it hits them in a vital part or breaks a leg, will be sufficient to put the brake on these magnificently brave people. ["For Foreign Service: Hints on Soldiering in the Shiny East," London, 1915]
Pile on the brown man's burden
And if ye rouse his hate,
Meet his old fashioned reasons
With Maxims up-to-date;
With shells and dum dum bullets
A hundred times make plain,
The brown man's loss must ever
Imply the white man's gain.
[Henry Labouchère, from "The Brown Man's Burden," 1899]
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