"to trim, to dress up," late 14c., perhaps a variation of Middle English proynen, proinen "trim the feather with the beak" (see prune (v.)); or perhaps from Old French poroindre "anoint before," and Old French proignier "round off, prune." Middle English prene (from Old English preon, a general Germanic word) meant "to pin," and probably influenced the form of this word. Watkins, however, connects it with Latin unguere "to smear, anoint."
Because of the popularity of falconry, bird activities formerly were more closely observed and words for them were more precise in English than today.
Youre hawke proynith and not pikith and she prenyth not bot whan she begynnyth at hir leggys, and fetcheth moystour like oyle at hir taill. ["Book of St. Albans," 1486]