If you read the section on etymology of the business term, it was referring to the metric of quality control. Basically it means your QC is so good that a bad unit gets shipped only a tiny fraction of the time.

Processes that operate with "six sigma quality" over the short term are assumed to produce long-term defect levels below 3.4 defects per million opportunities (DPMO). The 3.4 dpmo is based on a "shift" of ± 1.5 sigma explained by Mikel Harry.