Get down to brass tacks
Engage with the basic facts or realities.
The figurative expression 'getting down to brass tacks' isn't particularly old as phrases go. Its first appearance in print that I can find, from the USA in January 1863, was in the Texas newspaper The Tri-Weekly Telegraph:
"When you come down to 'brass tacks' - if we may be allowed the expression - everybody is governed by selfishness."
All of the other known early citations either originate in, or refer to, Texas. It is reasonable to assume that the phrase was coined there, in or about the 1860s.
The derivation of 'getting down to brass tacks' is uncertain. Nevertheless, it is a phrase that is often asked about, so I will list the most likely possible sources and the evidence for and against them and leave you to make up your mind for yourself.
Brass tacks are, of course, real as well as figurative items and two of the most commonly repeated supposed derivations refer to actual tacks. Firstly, there's the use of brass-headed nails as fabric fixings in the furniture trade, chosen on account of their decorative appearance and imperviousness to rust. Such brass tacks were commonly used in Tudor furniture and long predate the use of the phrase, which would tend to argue against that usage as the origin - why wait hundreds of years and then coin the phrase from that source? The supporters of that idea say that, in order to re-upholster a chair, the upholsterer would need first to remove all the tacks and fabric coverings, thus getting down to the basic frame of the chair. While that is true, it hardly seems to match the meaning of the expression, as the tacks would be the first thing to be removed rather than the last.
The second explanation that relies on actual tacks comes from the haberdashery trade. Here the notion is that, in order to be more accurate than the rough-and-ready measuring of a yard of material by holding it out along an arm's length, cloth was measured between brass tacks which were set into a shop's counter. Such simple measuring devices were in use in the late 19th century, as is shown by this piece from Ernest Ingersoll's story The Metropolis of the Rocky Mountains, 1880:
"I hurried over to Seabright’s. There was a little square counter, heaped with calicoes and other gear, except a small space clear for measuring, with the yards tacked off with brass tacks."
Various other explanations relate to the tacks in boots, those that were put on chairs as a prank, the rivets on boats etc, etc. None of these come equipped with any real evidence and are best left alone.
Of the supposed explanations that don't have literal allusions, we can rule out links with any form of 'brass tax'. There have been taxes on brass at various times, but no one can find any connection with this phrase. 'Getting down to brass tax' appears to be just a misspelling. The expression is also often said to be an example of Cockney rhyming slang, meaning 'facts'. In the strange world of Cockney argot, 'tacks' does indeed rhyme with 'facts' (facks), but that's as far as it goes. Rhyming slang coinages from the 19th century are limited to the UK and Australia. The apparent US origin of the phrase discounts the rhyming slang origin.
For my money, the 'fabric measuring' derivation is the strongest candidate but, given no smoking gun, we await further evidence.
See other phrases that were coined in the USA.