foil (v.1) Look up foil at
c. 1300, foilen "to spoil a trace or scent by running over it" (more commonly defoilen), irregularly from Old French foler, fuler "trample on, injure, maim; ill-treat, deceive, get the better of" (13c., Modern French fouler), from Vulgar Latin *fullare "to clean cloth" (by treading on it), from Latin fullo "one who cleans cloth, a fuller," which is of unknown origin. Compare full (v.).

Hence, "to overthrow, defeat" (1540s; as a noun in this sense from late 15c.); "frustrate the efforts of" (1560s). Related: Foiled; foiling. Foiled again! as a cry of defeat and dismay is from at least 1847.
foil (n.) Look up foil at
"very thin sheet of metal," early 14c., foile, from Old French foil, fueill, fueille "leaf; foliage; sheet of paper; sheet of metal" (12c., Modern French fueille), from Latin folia, plural (mistaken for fem. singular) of folium "leaf" (see folio).

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 sense of "metallic food wrap" is from 1897.
foil (v.2) Look up foil at
"apply foil to," 1610s, from foil (n.1).