early 15c., prouyne, from Old French proignier "cut back (vines), prune" (Modern French provigner), of unknown origin. Perhaps [Watkins] from Gallo-Roman *pro-retundiare "cut in a rounded shape in front," from pro "forth" (see pro-) + *retundiare "round off," from Latin rotundus (see round (adj.)). Klein suggests the Old French word is from provain "layer of a vine," from Latin propago (see prop (n.1)).
Or the Middle English word might be identical with the falconry term proinen, proynen "trim the feather with the beak" (late 14c.), source of preen [Barnhart]. Related: Pruned; pruning. Pruning hook is from 1610s; pruning knife from 1580s.