Wordorigins.org

Deadline is currently almost exclusively used to mean a time by which a task must be accomplished, but this was not always so. In the past, deadline had a variety of meanings, all related to a boundary for which there was a severe penalty for crossing.

The oldest of these uses dates to the American Civil War and refers to a line drawn around a military prison outside of which a prisoner could be shot, a literal “dead” line. From the Congressional Record of 12 January 1864:

The “dead line,” beyond which the prisoners are not allowed to pass.

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)