Plum comes through Old English plūme from Latin prunum and Greek proumnon. The fruit noun prune also reaches Latin prunum, through French.
Prune is a related borrowing, not an intermediate stage in plum. The pruning verb has a separate history. Middle English is omitted as a separate card.
Sources
- Merriam-Webster: plum
Word History gives the Greek, Latin and Old English forms.
- Merriam-Webster: prune
Noun Word History explicitly traces the fruit word through Anglo-French to Latin prunum and cross-references plum.
- Bosworth-Toller Anglo-Saxon Dictionary · search
Meanings of the older forms on the word path.
- logeion.uchicago.edu · προῦμνον
Meanings of the older forms on the word path.
- logeion.uchicago.edu · prunum
Meanings of the older forms on the word path.