September is Etymology Month (7)

foil "thin sheet of metal," early 14c., from O.Fr. fueille "leaf," from L. folia "leaves," pl. (mistaken for fem. sing.) of folium "leaf". The sense of "one who enhances another by contrast" (1580s) is from the practice of backing a gem with metal foil to make it shine more brilliantly. The meaning "light sword used in fencing" (1590s) could be from this sense, or from foil (v.). The modern sense of "metallic food wrap" is from 1946.