It was itself a 1e legacy term (or earlier), from when for instance the negative energy plane was originally the negative material plane, thus calling the normal material plane the Prime material plane. Planescape carried this into 2e, usually just calling it the Prime for short. 3e dropped the prime bit for reasons I don't particularly understand. I'm not even sure if 4e uses it the term at all, since I've seen it simply refer to it as "the world" (which I find bizarre since it would seem to imply one a single planet Prime, which is gimpy).