Английский Этимологический словарь - prune (v.)

- 16c., from M.E. proinen "trim the feather with the beak" (of birds), possibly from O.Fr. proignier "cut back, prune," from Gallo-Romance *pro-retundiare "cut in a rounded shape in front," from pro- "forth" + *retundiare "round off," from L. rotundus (see round). Related to preen.