Folkfact
The term folktale refers to a story passed down from one generation to the next.
I suggest the creation of a new term: folkfact.
A folkfact is a fact that is handed down through generations. The accuracy of a folkfact is often not questioned, and its origin is unclear.
Examples:
Rusty objects cause tetanus
Mongol warriors, knowns as Tartars, invented steak tartare by tenderising meat beneath their saddles.
Folkfacts are similar to common misconceptions, for which Wikipedia provides a comprehensive list.