Here are some options, depending on the context:
* Mustard: This is the most common and general translation, as "poupoun" often refers to a small jar of mustard.
* Condiment: This is a broader term that encompasses any sauce or relish used to flavor food.
* Small jar: This is a literal translation, if you're talking about the container itself.
* Small container of mustard: This is a more specific translation if you're talking about a specific type of poupon.
Ultimately, the best English word for "poupoun" will depend on the specific context and what you are trying to convey.