Wordorigins.org

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 1890s and that it is American in origin. Beyond that, there is only speculation.

The earliest known citation is from an 1895 letter by Frederick Remington:

How little I know...when you get down to brass-tacks.

The most likely explanation is that brass tacks is rhyming slang for facts. 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.

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.

Source: Oxford English Dictionary, 2nd Edition; Historical Dictionary of American Slang; New Partridge Dictionary of Slang)