Foil
Middle English (in the sense ‘trample down’): perhaps from Old French fouler ‘to full cloth, trample’, based on Latin fullo ‘fuller’. Compare with full2.
wiktionary
From Middle English foil, foille, from Old French fueille(“plant leaf”), from Late Latin folia, the plural of folium, mistaken as a singular feminine. Doublet of folio and folium.
From Middle English foilen(“spoil a scent trail by crossing it”), from Old French fouler(“tread on, trample”), ultimately from Latin fullō(“I trample, I full”).
From French foulis.
From mnemonic acronym FOIL(“First Outside Inside Last”).
See file.
etymonline
foil (v.1)
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.)
"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" (from PIE root *bhel- (3) "to thrive, bloom").
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)
"apply foil to," 1610s, from foil (n.1).
