Cashew nuts aren't really nuts...they're drupes.
It turns out there is all sorts of confusing classification and nomenclature regarding what I've long thought of simply as nuts. Cashews aren't nuts. Peanuts aren't nuts. Almonds aren't nuts. Acorns are classified as nuts but not many people eat acorns the same way people eat cashews or peanuts or almonds.
By definition a drupe is a fruit that's fleshy on the outside but with a shell covering the outside of the seed of the fruit - the cashew in this case.
A very different thing about a cashew is that although the seed is, indeed, covered with shell...in the case of the cashew, a really hard shell...the seed isn't on the inside of the fruit. Instead the cashew seed hangs from the bottom of the fruit, one per. My guess is that over the eons the cashew seed shell got really hard since it's not on the inside of the fruit that produces it. Protection, you know.
Peaches are drupes and the peach seed is very much an "innie." People eat the outside of the peach, the fleshy part of the drupe; but I doubt there are a lot people who eat the seed that's inside. With a cashew, the seed is an "outie" and hangs from the bottom of the fleshy part of the drupe. And the fleshy part of the cashew is commonly called a "cashew apple."
The common naming of the cashew fruit as an apple just adds more confusion to classification and names since the cashew is not at all kin to an apple. They're not even in the same family. The cashew tree family is Anacardiaceae while the apple tree family is in the Rosaceale. (This is really important stuff here.)
At False Bluff we have trees that produce both red and yellow cashew apples which make for really good eating. However, like the fruit of the pawpaw tree, a US native, a cashew apple is very fragile...too fragile to make it to a grocery store shelf in any appetizing way.
So we just pick and eat the cashew apple right off the tree. And like the fragile pawpaw, the apples of the cashew tree make wonderful jams and jellies.
We simply don't bother with the cashew seeds though....those things that are labeled "nuts" and are so very expensive in grocery stores. There are several reasons for the cost of the things - but that's another story.