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.
Another possibility is: In the 1860's 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". The tax, put into place by the U.S. government, was received as a direct attack on Southern slave owners, who relied heavily on brass products for daily functions on the plantation. "Get down to brass tax" was initially used to mean "stoop to someone's level" or "deal a low blow" in reference to the South's interpreted "cheap shot" from the U.S. government. After the war, however, it took on new meaning in reference to the brass tacks in a soldier's shoes, as mentioned above.
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 expression might also be rhyming slang for "facts