 The "Online Etymology Dictionary" presents the customary derivations of plum and prune from Latin "prūnum", the plum fruit.The tree is "prūnus"; and Pliny uses "prūnus silvestris" to mean the blackthorn.The word is not native Latin, but is a loan from Greek προῦνον ("prounon"), which is a variant of προῦμνον ("proumnon"), origin unknown.