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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