You may have thought the term “to pass mustard” was “to pass muster.” After all, muster refers to a military roll call, so it’s a common misconception that to “pass muster” means to meet or exceed a particular standard.
However, the term “to pass mustard” actually comes from ancient Roman times, where must, or the liquid part of crushed grapes in winemaking, was mixed with crushed mustard seeds to make mustum ardent, aka must ard.
Mustard making was a big deal, and due to the volatility of the must and a lack of refrigeration, when done poorly would make someone incredibly sick with the potential of death. So to “pass mustard” meant to literally be passing the bar for safe consumption.
Also, context matters, so make sure you’re always checking sources.