Wordorigins.org

A caucus is a meeting in which leaders and insiders set the agenda and policy of a larger organization or select candidates for office. It also can be used as a verb meaning to meet in a caucus.

The etymology is uncertain and there are several competing hypotheses. It has been claimed to date to before 1736, but the first recorded use of the term is from 1763 in John Adams’s diary:

This day learned that the caucus club meets, at certain times, in the garret of Tom Dawes.

This term is American, but of unknown origin, referring to a vague system of merit points used to curry favor with some authority. Anecdotal evidence indicates that it was part of military slang during WWII, but the earliest known use in print is from the Los Angeles Times of 15 March 1951:

You don’t know about brownie points? All my buddies keep score. In fact every married male should know about ‘em. It’s a way of figuring where you stand with the little woman—favor or disfavor. Started way back in the days of the leprechauns, I suppose, long before there were any doghouses.

Brothel derives, through the Middle English broþel, from the Old English bréoðan, meaning ruined or degenerate. It is a variant of the word brethel, meaning a good-for-nothing, a wretch.

The original sense was of a worthless or degenerate person and first appears in William Langland’s Piers Plowman, A text, which was written c.1367-70, with the surviving manuscripts dating to c.1390:

For nou is vche Boye, Bold Broþel, an[d?] oþer, To talken of þe Trinite, to beon holden A syre.
(For now is each boy (commoner), bold brothel, and other, to talk of the Trinity, to be looked upon as a sire.)

The phrase get down to brass tacks is of uncertain etymology. No one knows why it was originally coined, but there are several explanations. What we do know is that the phrase dates to at least the 1860s and that it is American, possibly Texas to be specific, in origin. Beyond that, there is only speculation.

The earliest known citations are from newspapers, the first being from the Houston, Texas Tri-Weekly Telegraph of 21 January 1863:

When you come down to “brass tacks"—if we may be allowed the expression—everybody is governed by selfishness.

Another early published use is from the Bangor, Maine Daily Whig & Courier of 12 January 1867:

The Galveston Bulletin says that Texas must “come down to brass tacks” and accept the constitutional amendment, unless the people wish Congress to proceed with reconstruction.

It is commonly asserted that brass tacks is Cockney rhyming slang for facts. It definitely is not British in origin, but it could be rhyming slang other than Cockney. This, however, is complicated by the variant brass nails, which dates to at least 1911. The variant doesn’t fit the rhyming slang, but then it may have been an alteration by someone who didn’t understand the rhyming slang. In any case, the rhyming slang explanation doesn’t appear until 1960, nearly a century after the appearance of the phrase, and is likely an after-the-fact attempt to make sense of the phrase.

Another explanation is that stores used to mark out a yard on the counter with brass tacks so that customers buying cloth could measure it by getting down to brass tacks and ensure they weren’t being cheated.

Yet another is that brass tacks were used as a foundation for upholstery. So getting down to brass tacks meant getting down to basics.

Sources: ADS-L; Oxford English Dictionary, 2nd Edition; Historical Dictionary of American Slang; New Partridge Dictionary of Slang)

The adjective blue has been associated with despondency and sadness since the 16th century. The noun the blues has been with us since 1741, when English actor David Garrick penned the following in a letter:

I am far from being quite well, tho not troubled w^th y^e Blews as I have been.

The blues is a shortening of blue devils, demons popularly thought to cause depression and sadness. Blue devils have been around since at least 1616, from Times’ Whistle, a collection of satirical poems from that year:

Alston, whose life hath been accounted evill, And therfore cal’de by many the blew devill.

The glossary to that work has an entry for:

Devil, blew devill, 107/3443. “Blue devils,” the “horrors,” or the remorse which frequently follows an ill course of life.

The name of the musical style has been around since 1912, taking its name from the mournful and haunting nature of the lyrics. Some sources say the style takes its name from the blue notes that it uses, blue notes being a minor interval in place of a major, an off-pitch note. But the opposite is true. Blue notes get their name from the blues, not the other way around. Blue note is attested to in 1919.

(Source: Oxford English Dictionary, 2nd Edition; Google Books)