A common name for Apios americana, a perennial vine with edible tubers (and edible flowers).
Etymology: loanword from Unami.
Also called "groundnut", but that name gets used for various plants so it can get confusing.
Here's an article about foraging and eating hopniss. This one includes recipes.
Always cook hopniss before eating it.
The plant has a long history as food, but is still rarely cultivated.
For Hunger Games fans, 'katniss' is also a Native American loanword that refers to an edible plant (Sagittaria sagittifolia).