The Collaborative International Dictionary of English v.0.48:

Subtile \Sub"tile\, a. [L. subtilis. See Subtile.]
   1. Thin; not dense or gross; rare; as, subtile air; subtile
      vapor; a subtile medium.
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   2. Delicately constituted or constructed; nice; fine;
      delicate; tenuous; finely woven. "A sotil [subtile]
      twine's thread." --Chaucer.
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            More subtile web Arachne can not spin. --Spenser.
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            I do distinguish plain
            Each subtile line of her immortal face. --Sir J.
                                                  Davies.
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   3. Acute; piercing; searching.
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            The slow disease and subtile pain.    --Prior.
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   5. Characterized by nicety of discrimination; discerning;
      delicate; refined; subtle. [In this sense now commonly
      written subtle.]
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            The genius of the Spanish people is exquisitely
            subtile, without being at all acute; hence there is
            so much humor and so little wit in their literature.
            The genius of the Italians, on the contrary, is
            acute, profound, and sensual, but not subtile; hence
            what they think to be humorous, is merely witty.
                                                  --Coleridge.
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            The subtile influence of an intellect like
            Emerson's.                            --Hawthorne.
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   5. Sly; artful; cunning; crafty; subtle; as, a subtile
      person; a subtile adversary; a subtile scheme. [In this
      sense now commonly written subtle.]
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   Syn: Subtile, Acute.

   Usage: In acute the image is that of a needle's point; in
          subtile that of a thread spun out to fineness. The
          acute intellect pierces to its aim; the subtile (or
          subtle) intellect winds its way through obstacles.
          [1913 Webster] -- Sub"tile*ly, adv. --
          Sub"tile*ness, n.
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The Collaborative International Dictionary of English v.0.48:

Subtle \Sub"tle\, a. [Compar. Subtler; superl. Subtlest.]
   [OE. sotil, subtil, OF. soutil, later subtil, F. subtil, L.
   subtilis; probably, originally, woven fine, and fr. sub under
   + tela a web, fr. texere to weave. See Text, and cf.
   Subtile.]
   1. Sly in design; artful; cunning; insinuating; subtile; --
      applied to persons; as, a subtle foe. "A subtle traitor."
      --Shak.
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   2. Cunningly devised; crafty; treacherous; as, a subtle
      stratagem.
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   3. Characterized by refinement and niceness in drawing
      distinctions; nicely discriminating; -- said of persons;
      as, a subtle logician; refined; tenuous; sinuous;
      insinuating; hence, penetrative or pervasive; -- said of
      the mind; its faculties, or its operations; as, a subtle
      intellect; a subtle imagination; a subtle process of
      thought; also, difficult of apprehension; elusive.
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            Things remote from use, obscure and subtle.
                                                  --Milton.
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   4. Smooth and deceptive. [Obs.]
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            Like to a bowl upon a subtle ground [bowling
            ground].                              --Shak.
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   Syn: Artful; crafty; cunning; shrewd; sly; wily.

   Usage: Subtle is the most comprehensive of these epithets and
          implies the finest intellectual quality. See Shrewd,
          and Cunning.
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WordNet (r) 3.0 (2006):

subtle
    adj 1: difficult to detect or grasp by the mind or analyze; "his
           whole attitude had undergone a subtle change"; "a subtle
           difference"; "that elusive thing the soul" [syn:
           elusive, subtle]
    2: able to make fine distinctions; "a subtle mind"
    3: working or spreading in a hidden and usually injurious way;
       "glaucoma is an insidious disease"; "a subtle poison" [syn:
       insidious, pernicious, subtle]