Alternative names, like groundnut and earthnut, make sense. In German, peanuts are called Erdnüsse, literally, earth nuts.
Where did the word "peanut" come from, and how did it become the dominant English name for Arachis hypogaea?
To make it short, peanuts are actually not nuts, but related to peas. So when a pea looks and tastes like a nut, what might one call it?
It used to be called both ground pea and ground nut. The etymology authorities at hand don't specifically say why peanut became popular. I suspect that if one pressed the matter further, one would receive more speculation than anything else.
My personal speculation is that peanut is easier to say and catchier as a word than ground pea or ground nut.
Sorry - too long for a comment, but I couldn't resist adding the famous piece of English Language and Usage:
Etymonline suggests earlier names were ground nut and ground pea, presumably both by partial analogy; I am more familiar with the former.
So perhaps somebody put the two together and then decided the ground was not necessary.
Thank you for your interest in this question.
Because it has attracted low-quality answers, posting an answer now requires 10 reputation on this site.
Would you like to answer one of these unanswered questions instead?