Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. Cunningly devised; crafty; treacherous; as, a subtle
      stratagem.

   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.

            Things remote from use, obscure and subtle.
                                                  --Milton.

   4. Smooth and deceptive. [Obs.]

            Like to a bowl upon a subtle ground [bowling
            ground].                              --Shak.

   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}.