What is a mustard snack?

"Mustard snack" isn't a commonly used term. It's possible you might be thinking of one of these:

* Mustard on Crackers/Bread: A simple snack where mustard is spread on crackers or bread, often with other toppings like cheese or meats.

* Mustard as a Dip: Mustard can be used as a dip for vegetables like carrots, celery, and pretzels.

* Mustard-Flavored Snacks: There are some commercially produced snacks with mustard flavors, like mustard-flavored chips or pretzels.

* Mustard as a Condiment for Other Snacks: You could use mustard as a condiment with other snacks like cheese and crackers, sausage, or hard-boiled eggs.

If you have more context, like what you saw or heard that made you think of a "mustard snack," I could give you a more specific answer.