The phrase originates from old tailor shops. Tailors would estimate cloth lengths by using their arms. Arm lengths vary, so clothing sizes would vary. When somebody demanded an accurate measurement, the tailor would use the most uniform measurement on their workbench: a row of brass tacks.
dw !p9hU6ckyqw joined in and replied with this 5 years ago, 4 minutes later, 2 days after the original post[^][v]#1,150,558
I thought they used the bras tax to buy brass tacks for getting down
blom replied with this 5 years ago, 8 hours later, 2 days after the original post[^][v]#1,150,720