Brass tacks is an object used in the popular expression "get down to brass tacks". The expression usually means clearing out confusing details and finding out the real facts about something.
The etymology of the expression is unclear. It may have roots in the way fabric manufacturers used to mark out a yard in tacks on the counter so customers could buy their fabric accordingly. It was common for some customers buying material or draperies to say to the salesperson "Let's use the brass tacks" which were embedded in the counter. Usually the salesperson would use his or her arm to measure the material. This has been seen in the UK even in the 1950s. This was done to save time overall, but the suspicious customer would often insist on having the material measured exactly, right up to the line of brass tacks.
Another possibility is that in the 1860s the US government issued boots for soldiers that were constructed using brass tacks to hold the leather soles on to the bottoms of their boots. As the boots wore down, the tacks would protrude through the sole and in to the bottom of the soldier's feet. 'Brass tacks' could mean to get to the absolute bottom of things in reference to shoes.
It is also argued that the idiom is derived from the "Brass Tax of 1854". When the makers of clothing, shoes, instruments, tools, etc. that required brass would gather the materials and count up the cost, accounting for the brass tax was the last - and most expensive - step. Therefore the phrase "get down to brass tax" could mean to get to the last and final thing, or to get past the formalities and get down to the crux of the matter.
It is also noteworthy that the tax, in addition to creating revenue for the government, led to a sharp increase in the cost of many instruments. tubas, trumpets, cornets, french horns, and other popular brass instruments gave way to flutes, piccolos, clarinets and oboes as the more affordable woodwind instruments' popularity skyrocketed. Evidence of this is most notable when examining Civil War marching music which relies heavily on the beating of percussion instruments and melodies from the woodwind family. Brass instruments are noticeably absent.
The earliest known use of the complete phrase in print, in the March 4, 1871 issue of the Galveston News (page 3), is "filing down to brass tacks"; hence, a shoemaker filing away too much material in excessive zeal to do a thorough job. The meaning was originally about the same as "putting too fine a point on it" or "over arguing the point."
The expression might also be rhyming slang for "facts."
The expression "down to the brass tacks" may have originated from a similar British expression: "down to the crass facts." This original phrase is used to describe dealing with the basic details. Because of the cockney British accent, it was incorrectly pronounced "down to the brass tacks," but still holds the original meaning.