The Collaborative International Dictionary of English v.0.48:

Foil \Foil\ (foil), v. t. [imp. & p. p. Foiled (foild); p. pr.
   & vb. n. Foiling.] [F. fouler to tread or trample under
   one's feet, to press, oppress. See Full, v. t.]
   1. To tread under foot; to trample.
      [1913 Webster]

            King Richard . . . caused the ensigns of Leopold to
            be pulled down and foiled under foot. --Knoless.
      [1913 Webster]

            Whom he did all to pieces breake and foyle,
            In filthy durt, and left so in the loathely soyle.
                                                  --Spenser.
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   2. To render (an effort or attempt) vain or nugatory; to
      baffle; to outwit; to balk; to frustrate; to defeat.
      [1913 Webster]

            And by ? mortal man at length am foiled. --Dryden.
      [1913 Webster]

            Her long locks that foil the painter's power.
                                                  --Byron.
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   3. To blunt; to dull; to spoil; as, to foil the scent in
      chase. --Addison.
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The Collaborative International Dictionary of English v.0.48:

Foil \Foil\, n. [OE. foil leaf, OF. foil, fuil, fueil, foille,
   fueille, F. feuille, fr. L. folium, pl. folia; akin to Gr. ?,
   and perh. to E. blade. Cf. Foliage, Folio.]
   1. A leaf or very thin sheet of metal; as, brass foil; tin
      foil; gold foil.
      [1913 Webster]

   2. (Jewelry) A thin leaf of sheet copper silvered and
      burnished, and afterwards coated with transparent colors
      mixed with isinglass; -- employed by jewelers to give
      color or brilliancy to pastes and inferior stones. --Ure.
      [1913 Webster]

   3. Anything that serves by contrast of color or quality to
      adorn or set off another thing to advantage.
      [1913 Webster]

            As she a black silk cap on him began
            To set, for foil of his milk-white to serve. --Sir
                                                  P. Sidney.
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            Hector has a foil to set him off.     --Broome.
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   4. A thin coat of tin, with quicksilver, laid on the back of
      a looking-glass, to cause reflection.
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   5. (Arch.) The space between the cusps in Gothic
      architecture; a rounded or leaflike ornament, in windows,
      niches, etc. A group of foils is called trefoil,
      quatrefoil, quinquefoil, etc., according to the number of
      arcs of which it is composed.
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   Foil stone, an imitation of a jewel or precious stone.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Foil \Foil\, v. t. [See 6th File.]
   To defile; to soil. [Obs.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Foil \Foil\, n.
   1. Failure of success when on the point of attainment;
      defeat; frustration; miscarriage. --Milton.
      [1913 Webster]

            Nor e'er was fate so near a foil.     --Dryden.
      [1913 Webster]

   2. A blunt weapon used in fencing, resembling a smallsword in
      the main, but usually lighter and having a button at the
      point.
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            Blunt as the fencer's foils, which hit, but hurt
            not.                                  --Shak.
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            Isocrates contended with a foil against Demosthenes
            with a word.                          --Mitford.
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   3. The track or trail of an animal.
      [1913 Webster]

   To run a foil,to lead astray; to puzzle; -- alluding to the
      habits of some animals of running back over the same track
      to mislead their pursuers. --Brewer.
      [1913 Webster]

WordNet (r) 3.0 (2006):

foil
    n 1: a piece of thin and flexible sheet metal; "the photographic
         film was wrapped in foil"
    2: anything that serves by contrast to call attention to another
       thing's good qualities; "pretty girls like plain friends as
       foils" [syn: foil, enhancer]
    3: a device consisting of a flat or curved piece (as a metal
       plate) so that its surface reacts to the water it is passing
       through; "the fins of a fish act as hydrofoils" [syn:
       hydrofoil, foil]
    4: picture consisting of a positive photograph or drawing on a
       transparent base; viewed with a projector [syn: foil,
       transparency]
    5: a light slender flexible sword tipped by a button
    v 1: enhance by contrast; "In this picture, the figures are
         foiled against the background"
    2: hinder or prevent (the efforts, plans, or desires) of; "What
       ultimately frustrated every challenger was Ruth's amazing
       September surge"; "foil your opponent" [syn: thwart,
       queer, spoil, scotch, foil, cross, frustrate,
       baffle, bilk]
    3: cover or back with foil; "foil mirrors"

Moby Thesaurus II by Grady Ward, 1.0:

236 Moby Thesaurus words for "foil":
   Roscius, actor, actress, antagonist, antipode, antipodes, antipole,
   antithesis, antonym, background, background detail, bad guy,
   baffle, bafflement, balk, balking, barnstormer, beat, bilbo, bilk,
   blast, brave, broadsword, buffalo, cast down, challenge, character,
   character actor, character man, character woman, check, checkmate,
   child actor, circumvent, claymore, coat, coating, collop, confound,
   confounding, confront, confusion, contra, contravene, converse,
   counter, counteract, counterbalance, countercheck, countermand,
   counterpoint, counterpoise, counterpole, counterterm, counterwork,
   covering, cross, curb, cut, cutlass, dash, daunt, deal, deceive,
   decorative composition, decorative style, defeat,
   defeat expectation, defy, design, destroy, detail, disappoint,
   discomfit, discomfiture, disconcert, disconcertion, discountenance,
   diseur, diseuse, dish, disillusion, disk, disrupt, dissatisfy,
   dramatizer, elude, embarrass, epee, evade, falchion, faze, feeder,
   feuille, figure, film, flake, flap, flummox, foiling, fold,
   foreground detail, form, frustrate, frustration, get around,
   get round, give the runaround, give the slip, glaive,
   go one better, hamper, heavy, histrio, histrion, impede, ingenue,
   inverse, juvenile, knock the chocks, lamella, lamina,
   laminated glass, laminated wood, lamination, lap, layer, leaf,
   let down, matinee idol, membrane, mime, mimer, mimic, monologist,
   motif, mummer, national style, nonplus, nullify, obverse, offset,
   opposite, opposite number, ornamental motif, outfigure, outflank,
   outgeneral, outguess, outmaneuver, outplay, outreach, outsmart,
   outwit, overreach, pane, panel, pantomime, pantomimist, parry,
   pass the buck, patina, pattern, peel, pellicle, period style,
   perplex, plait, plank, plate, plating, playactor, player, ply,
   plywood, protean actor, rapier, rasher, rattle, rebuff, reciter,
   repeated figure, repulse, restrain, reversal, reverse, rout, ruin,
   saber, sabotage, safety glass, scale, scimitar, scotch, scum,
   setback, setoff, setting, sheet, skin, slab, slat, slice,
   soubrette, spike, spoil, stage performer, stage player, stonewall,
   stooge, straight man, stroller, strolling player, stump, style,
   table, tablet, tantalize, tease, the contrary, the other side,
   theatrical, theme, thespian, thwart, thwarting, touch, trouper,
   tuck, upset, utility man, veneer, victimize, villain, vis-a-vis,
   wafer



The Free On-line Dictionary of Computing (30 December 2018):

FOIL

   File Oriented Interpretive Language.  CAI language.

   ["FOIL - A File Oriented Interpretive Language",
   J.C. Hesselbart, Proc ACM 23rd National Conf (1968)].