plum: [OE] Plum and prune ‘dried plum’ are ultimately the same word. Their common ancestor was Greek proumnon, a word which originated somewhere in Asia Minor. This was later contracted to prounon, and borrowed into Latin as prōnum. Its plural prōna came to be regarded in post-classical times as a singular, and this is where English gets prune from, but prōna was also borrowed into prehistoric Germanic, and many of its descendants here have had their r changed to l (the two are close together phonetically) – hence German pflaume, Swedish plommon, and English plum. => prune
