As Mango.org explains, it is actually a "drupe," aka a "simple fleshy fruit that usually contains a single seed," and belongs to the cashew family. It is about 5,000 years old and originated in ...

Drupes are fruits that have a fleshy exterior surrounding a single seed encased in a hard shell. The part we consume—the cashew “nut”—is the seed of this drupe. The cashew tree ...