Etymology Dictionary

ACCOUTRE,  to equip.  (F.,—L.?)   Shak. has accoutred, Jul. Cæs. i. 2. 105.—F. accoutrer, accoustrer.  Cotgrave gives both forms, and explains accoustrer by 'to cloath, dress, apparell, attire, array, deck, trim.'  Marked by Brachet 'origin unknown.' [†]   ¶ The most likely guess is that which connects it with the O.F. 'cousteur, coustre, coutre,' the sexton or sacristan of a church (Roquefort).  One of the sacristan's duties was to have charge of the sacred vestments, whence the notion of dressing may have arisen.  If this be right, we may further suppose the O.F. cousteur or coustre to be a corruption of Lat. custos, which was the Med. Latin name for the sacristan of a church.  Custos seems to have been corrupted into custor, as shewn by the existence of the fem. form custrix, which see in Ducange.  From custorem was formed the O.F. cousteur.  Custor seems to have been further corrupted into custer, which would give the form coustre, like maistre from magister; this also accounts for G. küster, a sacristan.  In this view, coustrer would mean to act as sacristan, to keep the sacred vestments, and hence, to invest.  Der. accoutre-ment.

ADDENDA

I find O.F. acoutrer in the 12th century, which is earlier than any quotation given by Littré.   'Les hardeillons moult bien acoutre Desor son dos,' i.e. he (Renard) arranges the bundles very comfortably upon his back; Bartsch, Chrestomathie Française, 202. 23.