Meaning that a peanut is actually closer to being a pea (also a legume) or a lentil (another legume) than an actual nut, like hazelnuts and pecans.
I know, it's weird, because it has the word "nut" in "peanut," which makes you think — THIS IS A NUT. It's also part of mixed nuts. And like... WHAT IS HAPPENING.
But you'd be wrong, because most edible nuts grow on trees, whereas peanuts grow underground in pods, which make them legumes.
So when you're eating peanut butter, you're having a nut in the culinary sense — because we've coined it as a nut — but really, you're eating yourself some legumes. More like pea spread on toast, if I may.