foilyoudaoicibaDictYouDict[foil 词源字典]
foil: English has three separate words foil. The oldest, ‘thwart’ [13], originally meant ‘trample’. It probably comes via Anglo-Norman *fuler from Vulgar Latin *fullāre, a derivative of Latin fullō ‘person who cleans and bulks out cloth, originally by treading’ (whence English fuller [OE]). Foil ‘metallic paper’ [14] comes via Old French from Latin folium ‘leaf’ (source also of English foliage [15] and folio [16]).

It originally meant ‘leaf’ in English too, but that usage died out in the 15th century. The modern notion of ‘one that enhances another by contrast’ comes from the practice of backing a gem with metal foil to increase its brilliancy, (Latin folium, incidentally, goes back to an Indo-European *bhel-, an extended form of which, *bhlō-, produced English blade, bloom, blossom, and flower.) The source of foil ‘sword’ [16] is not known, although the semantic development of blade from ‘leaf’ to ‘cutting part’ suggests the possibility that a similar process took place in the case of foil ‘leaf’.
=> fuller; blade, bloom, blossom, flower, foliage[foil etymology, foil origin, 英语词源]
foil (v.1)youdaoicibaDictYouDict
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.)youdaoicibaDictYouDict
"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)youdaoicibaDictYouDict
"apply foil to," 1610s, from foil (n.1).