foil-- from fouler (from full as in to full cloth..) came to mean trample , defeat, frustrate! (Foiled Again!)

foil from folium (a blade, as in a blade or leave of grass) a thin piece sheet of metal.. (aluminum foil) -- the same under a paste (fake) jewels to make it appear brilliant.. or a person who does the same for a comedian..(a straight man)

foil-- a narrow sword.. origin unknown..